The crazing on a very old uke would make me think the finish is either lacquer or possibly shellac. If you know what you are doing, then both finishes are relatively easy to make new by stripping back and starting fresh. If you don't know what you are doing (and I'm asuming from your question you don't so would need to farm out the job) then it will most likely turn into a disaster, or an expensive outsource.
BUT really, why would you? You aren't going to gain anything from a new finish except shine. In fact you could very well loose something from the tone because inevitably the top will need a light to moderate sanding back to even out the color. Loosing some wood is going to change tone. Very old lacquer or shellac will have it's effect on tone.
You don't say in what way the ripple is oriented. Down the top or across. In either case I wouldn't be doing anything with it.
Across is most likely the back edge of a bridge patch telegraphing through. Down the top could be a brace. In both cases it would be due to string tension rotating the bridge and bellying up the top behind the bridge. The telegraphing through of those bracing members means that the top is thin and trying to smooth these ripples out is only going to make it thinner, thus compounding a problem that really is nothing to be worried about at all if it's just a small ripple. Especially in an old instrument. Leave it alone.
The patina of age is something that should be appreciated. Love the old girl for what she is.