How hard is it to replace my cold water valve stem in my shower

Not difficult at all and it might be repairable. Shut off your main and open both handles, remove the handle and the ring beneath it and then turn the stem counterclockwise to remove. When a stem goes bad it's the washer abd or the seat. Replacing the washer or flipping it over may be all that is needed. The seat can be refaced with a seating tool or your can remove the seat and install new.