The below is taken from https://www.maketecheasier.com/fix-frozen-mac-when-updating-macos.
The Software Update item has not returned to the Apple menu. Mac App Store’s Updates tab now only lists app updates. The old Mac App Store preference pane is no more; Reintroducing the Software Update preference pane was the smart thing to do. MacOS updates and other system updates are no longer delivered through Mac App Store. Sometimes, the update could be stuck in a lull but not completely frozen. Some update processes will take much longer than others, resulting in a seemingly stuck progress bar. We can verify that the system is still updating by pressing Command + L to bring up the estimated install time. If this doesn’t appear, still let the update sit for a. Note: MacBook, MacBook Pro, and MacBook Air must have the power adapter plugged in to automatically download updates. See also Install and uninstall apps from the internet or disc on Mac Get Mac software and updates Change Software Update preferences on Mac. 10 Solutions: Mac Won't Turn On After Update to Big Sur or Catalina. The “Mac Won't Turn On After Update” issue can be caused by different reasons, here we collect 10 feasible solutions to get it fixed. Whenever a problem occurs, restarting your device is always the simplest yet efficient way to fix it.
If it’s clear that your Mac update isn’t going anywhere, then hold down the power button to turn off your Mac. Once you’ve done this, boot your Mac into Safe Mode. There are several ways to do this, but the simplest is by holding the Shift key as it’s booting.
The Apple logo should appear on your screen with a loading bar, and after that you should boot into Safe Mode. From here, run the macOS update as you normally would to install the update – hopefully without a hitch.
A very quick and easy thing to do that could save you a lot of hassle with stuck updates (or other malfunctioning Apple services) is to go to the official Systems Status page. Here you’ll see the status of just about every Apple service you can think of. Look over at the status of “macOS Software Update” to see whether the update servers are working as they should. If the bubble is green, then they’re fine and the issue lies elsewhere. If it’s red or yellow, then you’re just going to have to sit out the problem until servers are back to normal.
Speaking of sitting it out…
As painful as it sounds, you may just want to let the Mac sit for a couple of hours and see if it is indeed frozen. Sometimes, the update could be stuck in a lull but not completely frozen. Some update processes will take much longer than others, resulting in a seemingly stuck progress bar.
We can verify that the system is still updating by pressing Command + L to bring up the estimated install time. If this doesn’t appear, still let the update sit for a while.
Some users have reported their update taking upwards of ten hours. Grab a coffee, do some other work, run some errands, do whatever you need to do to burn some time and let your Mac do its thing. Big system updates (sometimes) take a whole lot of time!
If you are positive that the install is indeed frozen, you will want to take the following steps, and the following steps consecutively after this step, until the issue is resolved.
Let’s try to give the install a little kick by refreshing it.
If there is no avail, continue onto the next solution below.
NVRAM, as Apple describes it, is “a small amount of your computer’s memory that stores certain settings in a location that macOS can access quickly.” It also happens to hold kernel panic information, which is what we’re tapping into and especially trying to reset here.
Most of us are keen to update to a new version of macOS as soon as it is available. macOS Big Sur has been out since November 12, still, many users couldn't update their Macs straight away. It’s very frustrating if you see a message telling you that you don’t have enough space to install the new macOS. Don’t worry, though. There are lots of things you can do to fix it. We’ll show you what they are in this article.
While the installer for Big Sur takes up less than 16GB of space, you should make sure that you have at least 20GB of free space on your startup disk before you try to download and install it. The more free space, the better. Your Mac uses disk space to store temporary files and provide storage for active apps and tasks (which is what RAM does), so you should never be running with almost no disk space left.
CleanMyMac X makes it very easy to free up disk space on your Mac by identifying junk files and allowing you to get rid of them with a click. It can also detect large and old files and show you the files taking up most space and uninstall applications.
Complete the following steps to ensure your Mac is ready to update to Big Sur.
Back up all files and data that are stored on your Mac. Preferably twice to different locations. While upgrading to Big Sur is safe, you never know when something might go wrong. If you use Time Machine or another backup tool, run a manual backup just before you upgrade. And, if you can, clone your startup disk to an external drive, too.
If you don’t already have CleanMyMac X installed, download it for free here. This tool will help you clear some space and delete large and old files quickly.
Media files usually gobble up a large amount of disk space. And the worst thing is they are scattered around Mac’s folders, making it difficult to find and remove them. CleanMyMac X proves to be useful here:
Apple has provided a method for you to manage storage by showing you what is taking up disk space and making recommendations for deleting. It’s not as quick and easy as CleanMyMac X, but it can help you save disk space. Here’s how to use it.
There is one other thing you can try if you keep getting a message telling you that you do not have enough space to install Big Sur. You should only try it if you are sure there is enough space on your startup disk and that the message is wrong. And it will only work if you use Time Machine to back up your Mac.
Time Machine stores local snapshots on your Mac. Your Mac is supposed to count the space used by snapshots as free space, which is why it looks like you have enough space for the installation.
However, it seems that the installer may not realize the space is free. The solution is to delete the snapshots. This won’t remove backups on your backup disk, just snapshots on your Mac.
Open Time Machine from System Preferences. Uncheck Back Up Automatically and wait some time till Time Machine deletes snapshots. Then you can turn Time Machine back on.
If you’re familiar with Terminal, you can try deleting local snapshots manually.
tmutil listlocalsnapshots /
sudo tmutil deletelocalsnapshots
sudo tmutil deletelocalsnapshots 2020-10–09-002010
Hopefully, this helped you free up some space and update your Mac to the latest OS. If you're just preparing your Mac for the upgrade, don't forget to back up your data before you start clearing space. So you won't be afraid of losing anything important.