Once you have opened your controller to this point remove those 4 screws to separate the middle frame from the outer frame.

Once you have separated the middle frame from the outer remove the 2 screws securing the trigger assembly to the middle frame. Remove the opposite 2 if you are changing parts on the other side.

The trigger assembly comes out easy now.

Remove these 3 screws on this side of the trigger assembly and one more hidden behind the ribbon cable on the other side.

Your trigger assembly is now open. Gently remove the plastic piece to reveal more of the metal pin. Refer to second pic. Pull the metal pin holding the trigger out using a pliers and this will free up the assembly for the button film replacement.

The metal pin is now out and the trigger is free.

Remove the button conductive rubber and now replace the button film.

For reassembly reverse the order. Congratulations. You have successfully replaced your R1, R2 or L1, L2 button film.