IMac 21.5' 2017, Fusion drive, running Mojave 10.14.6. My update to Big Sur (following instructions in Take Control) seemed to be going fine until after about 40 minutes a progress bar froze before it got to the end, and stayed that way ever after. (This was yesterday.) Restarting just produces an apparently stationary progress bar again. Apple advises that macOS Mojave will run on the following Macs: Mac models from 2012 or later. If your Mac is more than a few years old and you try to install Big Sur on it you will hit a few.
macOS Big Sur elevates the most advanced desktop operating system in the world to a new level of power and beauty. Experience Mac to the fullest with a refined new design. Enjoy the biggest Safari update ever. Discover new features for Maps and Messages. And get even more transparency around your privacy.
Check compatibility
macOS Big Sur is compatible with the following computers. If your Mac isn't compatible, you might be able to install an old version of macOS instead.
MacBook introduced in 2015 or later
MacBook Air introduced in 2013 or later
MacBook Pro introduced in late 2013 or later
Mac mini introduced in 2014 or later
iMac introduced in 2014 or later
iMac Pro
Mac Pro introduced in 2013 or later
View the complete list of compatible computers.
If upgrading from macOS Sierra or later, your Mac needs 35.5GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44.5GB of available storage. To upgrade from OS X Mountain Lion, first upgrade to OS X El Capitan, then upgrade to macOS Big Sur.
Download macOS Big Sur
If you're using macOS Mojave or later, get macOS Big Sur via Software Update: Choose Apple menu > System Preferences, then click Software Update.
Or use this link to open the macOS Big Sur page on the App Store: Get macOS Big Sur. Then click the Get button or iCloud download icon.
Install Mojave On Big Sur
Install macOS Big Sur
After downloading, the installer opens automatically. Click Continue and follow the onscreen instructions.
Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, it might restart and show a progress bar or blank screen several times. You might find it easiest to begin installation in the evening so that it can complete overnight, if needed.
Stay up to date
Install Mojave On Big Sur Mac
After installing macOS Big Sur, you will be notified when updates to macOS Big Sur are available. To check for updates, choose Apple menu > System Preferences, then click Software Update.