How to change my iMac Apple ID

Do you mean your user account name? It can be done, I've successfully done it a couple of times, but you have to be very precise in doing it or you'll render the account useless. And if you have only one user account, well ..........

Change the name of your macOS user account and home folder - Apple Support

If you mean your Apple ID, you might want to consider creating a new one. But having two active Apple IDs can cause problems too.

Change your Apple ID - Apple Support