Choose the Time Machine backup disk>Continue.Choose Restore from Time Machine Backup from the macOS Utilities interface and click on Continue button.Hold down Command+R during mac rebooting.In this case, you should restore both your files and settings from a Time Machine backup. The only change you want for the downgrade is turning back to a previous macOS version, but you want the files and settings the same as before.
Connect the bootable drive to your mac.Create a bootable drive of macOS Catalina or Mojave.Reinstall macOS Catalina or Mojave from Bootable Drive Agree to the terms and follow the instructions to download and install.In the pop-op macOS Utilities interface, choose Reinstall macOS>Continue.Hold down Command+R when rebooting your mac.Reinstall macOS Catalina or Mojave from Internet When the download finishes, follow the on-screen instructions to install macOS Catalina or Mojave.Search macOS Catalina or Mojave, click Download.
There are 3 methods to install older macOS version on your mac.
It is safe to upgrade or downgrade a macOS version, there is no harm done to your files or data, but data loss after mac update or reversion happens sometimes. Also, make sure your mac is connected to the internet for later macOS installer download. You should quit running programs and leave enough space (15GB to 20GB) to install Catalina after erasing Big Sur from your Mac hard drive. Before the downgrade, your data matters the most, so don’t forget to back them up though it takes time to finish a backup. To downgrade Big Sur to Catalina without losing data, here are 6 steps to follow.
How to Downgrade Big Sur to Catalina without Losing Data? No matter what your reason is to revert to an older version, you can refer to following part to downgrade from Big Sur to Catalina without losing data. Your favorite app doesn’t work on the new version.Your hardware is not highly compatible on this new version (compromised audio, video or image quality).MacOS 11 Big Sur keeps causing problems.It takes longer time to get used to the new version and you don’t want to waste time.The Beta version is not perfect, you want to download the official version later.But later, there occurs problems and some of them want to revert to the previous Catalina version for one or another reasons: