How can I remove the previous owner’s account from a used MacBook Air?

You cannot.

This is what the previous owner should have done: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

This Mac is now forever associated with the previous owner. You will not be able to properly personalize it for your own use. This is an anti-theft and security feature. There is no workaround.

If you can, return it to whomever you bought it from. If you can't return it, then take this as a lesson. Never buy a used Mac from an unknown source. Buy only from Apple and reputable Mac resellers.

You’ll need to have the previous owner remove the account.