On the buttplate of the stock there are two rubber plugs. Pry those out with a screwdriver and then unscrew the buttplate. After that you need a long screwdriver to unscrew the stock from the receiver. Then unscrew the silver screw located infront of the trigger guard. Pull the trigger assembly out. Pull the pump back and then you can wiggle out the valve assembly.
To fix your leaking problem all you need to do is unscrew the valve from the back of the valve assembly and you should be able to find the damaged oring.
To make the adjustable hop up all you need to do is drill a hole in