What's the best way to clean a leather wallet?

Start by wiping it with a soft, damp cloth (not soaked) with a bit of neutral soap on the stains, then dry it well with another dry cloth. After that you can apply a leather cream or conditioner - like Kiwi or similar products - which will hydrate the material and leave it looking better.

First thing is to test any product on a discreet corner of the wallet so you don't damage it further. For dark stains, a damp cloth with a bit of neutral soap does the trick pretty well - go over it carefully and then dry it thoroughly. After that, apply a leather conditioner (like Meltonian, which you can find at any shoe store) to restore flexibility and protect against future damage. If the stains are really deep, a cream made for dark leather also helps disguise them while taking care of the wallet.

After cleaning with neutral soap, there's a trick that works really well: use diluted white vinegar in water (like 1 part vinegar to 3 parts water) with a well-wrung cloth on dark stains - it removes soap residue and helps lighten them. Let it dry naturally away from direct sunlight. Then apply a good quality leather conditioner to restore the oils the leather loses during cleaning. Guess uses decent leather, so with this treatment it'll look good as new.

If you're going to try the vinegar they mentioned, be careful not to get the leather too wet - keep it damp but never soaking, or you could warp the wallet or end up with even worse water stains. What nobody brought up is conditioning afterward: once you've cleaned and dried it thoroughly, apply a leather-specific cream or oil (like mink oil or actual leather conditioner) to keep it from drying out and getting brittle, especially for a Guess that's already a couple years old.