how can I go back to High Sierra after upgrade to Catalina
How can I go back to High Sierra from Catalina
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Downgrading your macOS to an earlier version is a difficult and tedious task. It requires that you:
1) Back up your Mac with Time Machine (or other backup solution) to an external disk drive, preferably not the same backup drive you were using for the previous OS version.
2) Have available an installer for the previous version. This can be a Time Machine backup made prior to your upgrade to the current version, or it can be a bootable installer for that previous macOS created on a USB flash drive. You can get old versions of macOS here. 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 wipe the drive in preparation of reinstalling the previous OS.
4) Reinstall macOS on your computer using your prepared USB installer or the Time Machine backup made with the older macOS. You may also use the macOS reinstaller that is offered at the macOS Utilities screen after booting in Recovery mode.