The whole gun has to come apart in order to get the valve out. Its quite simple to do really.
1. First remove rear breech screw and the small breech screw under the bolt in order to remove the whole breech and barrel from the barrel band. Also remove the transfer port and seal.
2. Remove trigger frame screws - front and back. The front one screws into the valve. Watch out for the tiny safety spring screw and ball bearing. Keep the pistol upright when you do this.
3. Next extend the pump arm and tap the pivot pin out using a punch and hammer. The barrel band can then be removed then with the piston.
4. Tap the piston pin out and replace with the flat top. You will need to adjust the piston when reassembling.
5. From the back of the pump tube - use a piece of dowel, I use a wooden chop stick to gently tap the valve out. The valve has to come out the front of the pump tube, so tap from the back.
With regards to adjusting the piston - you should feel the piston end meet the top of the valve when the pump arm is about 1/2inch from closed.
A heavier hammer spring will allow the valve to dump more air. Inside the valve, the valve return spring determines the amount of air taken in during each pump. The lighter the spring, the more air is taken. Also a lighter return spring means the hammer spring can dump more pumps.
I'd recommend a slighter lighter spring for your valve and a sdcustomdesign hammer spring and guide for smoother firing cycle.