I changed my battery on my iPhone xsmax at batteries plus. How do I get rid of the error notification about my non Apple battery?

You don’t. they are not an authorized service provider and do not have legitimate access to genuine Apple parts or the tools necessary to properly install and register them.

If you are very lucky, Apple will agree to replace the battery they put in it with a genuine battery, but don’t be surprised if they refuse to touch that device now that it’s been tampered with.

You don't. Batteries Plus is not an authorized Apple Service center and therefore they didn't use a genuine Apple battery. That message will remain there forever. Unfortunately, Apple will not work on it now that it has been tampered with by non-authorized personnel.