Back up any important files
Sign out of iCloud
Sign out of iMessage
Restart the MacBook Air
Immediately hold Command + R until the Apple logo or macOS Utilities appears
Select Disk Utility
Click Continue
Select Macintosh HD or the main internal drive
Click Erase
Choose APFS or Mac OS Extended (Journaled) if prompted
Confirm the erase
Quit Disk Utility
Select Reinstall macOS
Click Continue
Follow the on-screen instructions
After installation, shut down if you are selling or giving away the MacBook Air
If needed, reset NVRAM by restarting and holding Option + Command + P + R for about 20 seconds