There are three pins located behind the trigger. Two of them are held in place only by pressure, while one has a small black rubber band. Use a nail set that closely matches the size of the pins, and carefully hammer them out using the nail set.
Remove the trigger (the red part on my nailer) while noting how it connects to the trigger valve assembly, so you can reassemble it in the same manner later.
Take out the old trigger valve assembly using your fingers or a pair of pliers.
Apply pneumatic tool oil to lubricate the o-rings on the new trigger valve assembly.
Install the new trigger valve assembly. Then, reattach the trigger, ensuring the holes in the trigger align with the corresponding holes where the pins were located.
Using your hammer and nail set, gently tap the pins back into place, ensuring they are inserted in the correct positions, as they vary in size. Finally, reattach the black rubber band piece to the pin from which it was removed.