There isn't a single "administrator" on macOS. You can reset any user's password in various ways: Change or reset the password of a macOS user account - Apple Support
However, if you don't have another admin capable account, you can do this:
- Boot into Recovery.
- Run Terminal from the Utilities menu.
- type, resetpassword at the prompt and hit return. Follow the instructions to reset the password for whatever account you wish to change.