I forget which side is easier, but with a bit of work you can remove the easy side simple by putting a screw driver on the button and turning it counter clockwise about 120 degrees. That will release the cap and you can then use a tweezers to pull ot the plastic cap and then the needle. On the other side you have to remove the screw that hold the slide to the linkage. If I remember correctly, once you remove that you can push it out of the way and remove needle like the other side. It's a bit of surgery and you need to take care not to drop anything.

Andy might be along shortly with additional advice. You can do all of this with the carbs in the bike, I've done it at the track.

The stock needle is not adjustable so unless you are changing to HRC needles or believe you already have HRC needles, there isn't much to adjust. About all you can do is add a shim. I think the stock needle already have a shim under their top so be careful ful not to lose them.