Need help removing hot water knob in tub/shower

[Novice#10]

I’ve just purchased this house and the hot water barely comes out. I can remove the screw and knob but can fig out how to remove the outer part to get to the stem……. I’ve use clr and wd40 both and it isn’t budging with my wrench. I’ve attached a photo with red arrows pointing to the part I can’t remove.

 

[Slomoola]

Get a pipe wrench or water pump pliers. Try to tighten it all the way first. Clean the threads to assist in removing. Little spray juice or heat might help.