How do I remove a Stanadyne shower valve installed behind a fiberglass shower stall.

That is a Moen mixing valve. I can tell you how to remove it but since its carries a lifetme warranty I am wonedring if you like to fix rather than replace. Let me know.

I would like to fix it if possible.

Contact Moen for a 1225 cartridge for your valve. Warranty Information for Moen Faucets Shut down the main, remove the handle, small screw under handle, remove escutcheon plate, remove clip on top of valve and pull valve toward you. When the replacement cartridge arrives it will have instructions but its pretty much the reverse of what I gave yo to remove, grease the new cartrdige with plumbers grease.