As Morris said they are pretty easy to rebuild.
Separating the valve from the mounting bracket will reveal a set screw on the back side. Remove this screw and work the valve cylinder from the valve body. You’ll probably see a lot of crud and a worn out O-ring. Clean things up a bit, pick up a new O-ring (NAPA) and you’re good to go. I had to use a wire brush and emery cloth on mine being carful of the brass. You'll also want to lube things up when you reassemble the valve. I use Mercruser outdrive grease since it’s made for use around water and heat, but you could use plain old plumbers grease from the hardware store. Slide the valve back together; replace the set screw, and your done.
Just like that, you’re done