How do I remove Mojave?

Do you have a Time Machine backup to Restore to/from?

https://support.apple.com/guide/mac-help/revert-to-a-previous-macos-version-mh15216/mac

If not…

https://www.imore.com/how-downgrade-macos

Time machine is the best

i always go back to mountain lion on my other macbook air by time machine, or rebooting with a copy of the OSx on a thumb drive. You could download high sierra and reboot on your mac. Their are many steps on the apple website.

personally i love Mojave because the osx is extremely responsive, even on a 4gb ram macbook air.

i advised many apple users to upgrade their ram to 8 if possible who are now pleased they did..

good luck, hope this helped!