For now, unscrew the rod thing (Cocking rod) on the back of the marker, and stick a allen key in the hole where the rod came out. It can be anywhere from a 5/32 key, to a 7/32 key. Most of them are 3/16 though. Turn the key clockwise to raise the velocity, and counter clockwise to lower. Then screw the rod back in.
Heres a better guide to tuning a cocker when you have more time.