I inheritedmy dad's apple computer. How do I change the Admin name on the computer?

Above is correct.

That written, I’d erase and re-install macOS, once you have migrated all data and files and photos that you want to preserve.

And I’d update the Mac to current, if it’s not already running Monterey, and is able to run Monterey.

If you don’t do that reset, the existing contents are all tied to another Apple ID, which will cause ongoing problems when you try to update, and try to share storage via iCloud—Apple ties all of these to the Apple ID.

The procedure is basically this: What to do before you sell, give away, or trade in your Mac - Apple Support

If the computer is as left by your late father and you know his computer login you can do the procedure set out in the article below. This will create a new user (you) with admin privileges.

https://osxdaily.com/2017/07/17/how-create-new-admin-account-mac/#:~:text=Here%20are%20the%20steps%3A%201%20Go%20to%20the,%E2%80%9CCreate%20User%E2%80%9D%20to%20create%20the%20new%20Administrator%20