Can I just dissasemble the top LCD of the 3DS XL?

No you don't have to take the bottom half off. You actually don't have to take anything off other than the plastic LCD protector.

It's a trick to get it off, but it does come off. You will need a pick or something that can get between the case and the lens. What I use is a metal pick that I get get up under the plastic lens a little bit. I then pull the lens up slowly making sure not to break it or bend it too much.

You have to be very, very careful working this close to the LCD with tools as one slip and you could break it or scratch it severely.

When I get it off I usually just use canned air to get debris off. If there is some that doesn't come off you can use a microfiber cloth to clean it, but keep in mind that eve with a clean cloth it will still leave a residue that is very hard to get off. Canned air is best.

You may also consider just getting a new lens to put on it while you are at it.

I hope this helps!