looks to me like you should unscrew the grip frame, remove the hadn guard, replace it with your new one.
the trigger is probally held in with a pin you push out (be sure to push the star side/larger side out first) then you put your new trigger in.
it'll take time putitng it back in. before you take the orgional one out, be sure to take carefulnot of how everything goes together so you can duplicate it with your new one.