Need help with a leaking diverter.

Hi all. My first post. I am renovating an '80s condo and the diverter is leaking. I tried to unscrew the outer cover but it seems stuck.

I don't want to damage anything and hope to fix my self. I did get a qout to change out the three handel system to a one handel with a remodlers plate, but th e estimate was $900.

Any ideas of the type of valve and a compatible replacement?

Thank you