With action removed from stock remove cocking arm pivot pin from trigger block and lift arm clear of block. You can remove block and change mainspring but if the spring needs changing then the piston buffer washer will need doing on the later guns---SO. Remove pins from the small cocking arm link and the sliding plate. On some of the guns you will also have to remove the nut from the bolt (or the bolt) that holds the little support bracket and goes through the sliding plate. Remove the sliding plate and follow the Mercury guide for the strip down and replacing of the internals. Once you have done that it is just a matter of refitting the cocking arm set up, sliding plate,bracket and nut/bolt, cocking arm link and cocking lever.
Loading taps are different. The early ones had the lever as a part of them and the little spring and ball under the plate on the RHS (I think)---Later ones had a metal lever screwed on and the ball and spring in the tap at a right angle. It also had shims under the metal lever. Just watch and take care. If the gun does not fire a pellet out after you have done the strip the tap may be out of line and need a shim (on later ones) or the lever touching with a file on the inside face if catching on cylinder with earlier ones. Replace action in stock, tighten screws and refit sights.