How do I clean some paintings?

I have two paintings that I was given from a friend who painted them about twenty years back. I have no idea how to clean them off. I’m scared of wiping them off with a rag and water and a dry rag doesn’t seem to clean them that well. Do you have any suggestions? They aren’t dusty very much because I gently use a feather duster on them, but I want them to look a bit more cleaner you know?