OK, you should see 2 bolt heads, one holding the lift arm to the shaft that rotates to lift the needles and one at the top of the lift pivot assembly. Undo the one on the shaft and the lever will lift the slide freely. Lift it as far as possible and pop the pivot ball out of the side of the lift pivot assembly. You should be able to lift the lever out of the way and pull the slide right out. There are 2 screws holding the lifter to the top of the slide, remove these and the lifter turns a bit to come off revealing the top of the needle and the clip that keeps it in and sets the height. Your needles may have multiple grooves or just one; if several, putting the clip higher (nearer the flat end) makes a leaner mixture, closer to the pointy end makes it richer. If there's just one groove you can put a small washer under the clip and make it richer a bit.
Don't drop the tiny clip, they're pretty hard to find - on the floor or as a replacement part.
Reassembly is pretty much the reverse, if you can't gently persuade the ball to go back in the arm then remove the cap bolt, directly underneath is a (graphite?) thingy that presses on the top of the ball. Probably just loosening the cap screw will let you get the ball in easily but you can take that thingy out and get it ine with no hassle. Don't remove any of the parts underneath the "thingy", there's a few of them and the correct order of reassembly may not be obvious.
Doing this to the carbs will change balance slightly, you should get the needle height you are happy with then do a carb balance after the stack is together for the last time.
Remember the needle just affects from about 1/4 to 3/4 throttle mixture. The main jet does it all from 3/4 or so to full, and the bleed screw and pilot jet dominate below 1/4 throttle. Best practice is to do the main jet first (full throttle plug chops) then the needle (1/2 throttle chops) then the pilot jet and airscrew.
