Admin name and password change
How can I do this on a computer whose former owner doesn't know his password?
My OS is Mojave 10.14.6
Question marked as
Boot into Internet Recovery, Erase the drive, then reinstall Mac OS.
How to reinstall macOS from macOS Recovery - Apple Support
If you want to keep all of the cruft left by the previous owner, create a new admin-capable user and reset the password from there.