Smelly belly buttons are a common problem and even soap and water don't always help. People are embarrassed by this and sometimes they are afraid to ask around even though they keep clean and it doesn't seem to help. Try The Navel Fresh Spray, it's made just to clean the belly button! It smells great and it really helps.
I'd say, take a shower, keep it clean. That should help with the odor. Good luck with that.