What are the steps to replace the on/off volume button?

There is no iFixit guide for doing this but if you wanted to do it yourself, you could look at the guides for the previous generation of phones (iPhone 7/8). While the steps will be different, the general idea is the same for all the devices. It usually requires removing the battery (battery first!), screen and logic board to get access to the button flex.

If you’ve never done this before, give it proper consideration. It’s not necessarily difficult but errors can cause expensive damage. Go through the guides completely before opening the phone.