Make Tech Easier published a tutorial about how to downgrade macOS to a previous version.

How to Downgrade macOS to a Previous Version

While the latest version of macOS is one of the year’s most anticipated events, a small number of users face issues with their installations. Besides the usual bugs with fresh software releases, you may also run into problems like unresponsive systems. Fortunately, it’s possible to downgrade your system to an earlier version of macOS, such as the OS version that was shipped with your device. These steps will help you downgrade from an later version of macOS, such as Big Sur, to an earlier macOS version. 

How to Downgrade macOS to a Previous Version