Unwinterizing is pretty easy. Just have to make sure the bypass valves are in their normal in-use position then just run water through the system.
Usually when unwinterizing you'll also want to sanitize the system. That can be accomplished by running a sanitizing solution through the system (put some in the water tank, turn on the pump, run every faucet until you can smell it, turn off the pump, let it sit for a while, then drain everything and flush with fresh water).
A solution such as:
TastePURE Spring Fresh - Camco 40207 - Fresheners & Softeners - Camping World