How do I install a new diverter stem extension in my shower. Can't remove the old one.

First, what kind / brand / style of faucet do you have ? Can you post photo of it ?

How to install a diverter stem extension and hot stem for bath?

Jawill220: An "Extention" is used to extend existing stem. Sometimes, existing stem has to be extended due to too thick wall finish. There are few different way how to attach extension stem: it can be screw with phillips head, it can be small set screw on the side, and/or it can be screw-on piece. If you tell me what kind of faucet you have - or post a photo with detail of the stem you want to extend - than I will be more specific in my response.