The below is taken from https://www.maketecheasier.com/fix-frozen-mac-when-updating-macos.
1. Install the Update in Safe Mode
Discord Stuck Checking For Updates On Mac Os Sierra Ht208202
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.
HOW TO FIX DISCORD STUCK IN AN UPDATE FAIL LOOP WORKING 2018 (QUICK) - Discord Update Failed Fix - Duration: 0:56. Minute Manual 396,660 views. IS DISCORD REALLY THE BEST? Fixes to a Stuck Mac OS Sierra Installation Once you have checked everything and you are positive that your Mac has completely stalled, follow these steps: Restart your Mac. Press and hold the Power button to switch off your Mac. Wait for a few seconds and restart your Mac. Visit the Mac App Store.
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.
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2. Check the Apple Systems Status Page
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…
3. Sit 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!
4. Refresh the Update
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.
- Hold down the power button and wait for about 30 seconds.
- When the Mac is completely off, press and hold the power button again. Now, the update should resume.
- Press Command + L again to see if macOS is still installing.
If there is no avail, continue onto the next solution below.
5. Reset Your NVRAM
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.
- Press and hold the power button on your Mac to completely shut it off. Hard discs and fans need to stop spinning, and the screen needs to go dark.
- Power on your Mac.
- Immediately after you hear the startup sound, press and hold the Command + Option + P + R keys.
- Keep holding them down until you hear the start up sound again.
- Release the keys, and the NVRAM will have reset. The update should resume.
Tip & Tricks to Fix Mac Installation Failed/Stuck/Frozen/Slow Errors
If your Mac is unable to install macOS Big Sur or freezes on installing, you can try to fix it with the following tips. There is no guarantee that each tip works on macOS update problems. But it covers the common measures when Mac users fails to install new macOS, hopefully any of these tips will work for your macOS installation problem.
1. Shut down your Mac and wait for some time. When a new OS releases, a massive number of people will go to download and install the updates in a snap. As a result, the downloads and installation will be very slow with the large file and many people doing the same thing at the same time, and even fail during the process. What you should do is to wait for the server becoming less busy. According to Apple system status page, macOS Software Update issue has been resolved. Now you should download the macOS Big Sur update file faster and install it without seeing 'An error occurred while installing the selected updates'message.
2. Go to Mac App Store and open updates. If your Mac install stuck on Apple logo, gray/white/black screen when you were installing an App, you can go to Mac App Store to find this app and open the updates to survive from the macOS installation stuck situation.
3. Download & install the combo update. The Mac App Store isn't the only place to download macOS from. When your encounter problems on macOS Big Sur installing, you can try to download the combo macOS update from the Apple official site because the macOS installation package downloaded from Mac App Store may only include the essential files for your Mac update, instead of the combo updater with all the files required to update to macOS.