Just bought a house, and it was a flip house so seller has no idea on mailbox key is, how much to change up the lock, or how does that all work?

$40, we just did this this week.

Post office can help.

Save yourself alot of money, wait for mail to get delivered, ask postal worker to let you take the lock off your box, go to hardware store and replace said lock. Theres other options like contacting post office, HOA, or a lock smith if youre willling to pay their fees