How to revert to an older version of macOS

No, you really don't.

It's probably easier and wiser to simply work through any problems you are experiencing than trying to downgrade your macOS.

Downgrading your macOS to an earlier version is not a task to be taken lightly. It requires that you completely erase the data from your drive, reformat that drive, download and reinstall the older OS and finally recover your files from a backup made prior to your installation of the macOS Tahoe. 

Even then, the Mac’s firmware may have been updated, adding a potential for conflicts and a possible need for DFU mode and a Configurator revive/restore, which requires using another Mac connected to yours. 

If all that sounds complicated and risky, then you understand the reality.

Before you attempt this, cross your fingers, kiss the St. Christopher and hope all goes well on your journey.

1) Back up your Mac with Time Machine (or other backup solution) to an external disk drive.

2) Download the previous OS and create a bootable USB installer for it on a USB flash drive. You can get old versions of macOS here.  Test and make sure it is fit for use after you create it.  If it boots your Mac and launches the macOS installer, then it’s good. Quit the installer then continue to the next step.  Optionally, a previous version may be available on an Intel-based Mac if you use Shift-Option-Command-R startup key combination during startup. You may be offered the macOS that came with your Mac, or the closest version still available

3) Boot your computer in macOS recovery mode and launch Disk Utility. Erase and format the drive in preparation of reinstalling the previous OS. This step will erase all of your stored data.

4) Reinstall macOS on your computer using your prepared USB bootable installer that you  created.