If you can find out what the instrument was originally finished with, that would be a plus, although not entirely necessary. Most likely, it has a nitrocellulose finish, which means you can sand it down with some 220 sand paper, then re-spray. Pretty much anything you can find in a can at the local hardware store should be sufficient, for one uke. Just because it comes in a can, doesn't mean that you can't get professional results. It just takes some practice. Try shooting a few pieces of scrap lumber you can find around the house.
The second most likely finish is a catalyzed poly finish. It's still okay to shoot nitro(spray can laquer) over this, but not the other way around. Poly finishes dry harder than nitro and you may get separation between the coats, if the base is softer than the top.
Third most likely is that it's a urethane based finish. Still fine to shoot laquer over, for the same reasons as poly.
Although you can shoot right over after sanding, I would recommend stripping the old finish to the wood. You may encounter discoloration due to different finishes being applied. If you want to chance it, try a small test patch on the back of the head, to check it out.
Good luck!