How do I reset my password for my IMac 21.5 home computer?

Hi there!

If you forgot the password to log in to your Mac user account, or the password isn't accepted, choose a reset option from the login window.

If you forgot your Mac login password - Apple Support

If you have tried those steps and you cannot reset your password, or you're using macOS Mojave or earlier, see: If you can't reset your Mac login password - Apple Support.

Jack

Let us have some more clues, like exactly what steps are you taking where it fails. :)