How do I reinstall my operating system on MacBook Air

• How to reinstall macOS - Apple Support

https://support.apple.com/en-us/HT204904

(install options are shown in this article)

You might need to erase your disk first.

"The best way to restore your Mac to factory settings

is to erase your hard drive and reinstall macOS."

Be sure to have/had good backups to include

the Mac's Time Machine and others too.