In all likelihood, you just need to replace it. Nearly all helmets, climbing and bicycle helmets included, need to be replaced once they become worn or older than 5 years. Ideally, you would track how old each of your helmets are. If you don't know how old one is and it looks well used, I wouldn't risk it.

Stick to gently wiping it with a soft dry cloth. Trying to clean a helmet with water or most any kind of soap only risks damaging the foam. If a helmet is so worn that a dry cloth doesn't restore it, it might be too warn. Consider how important your head is before try to cover it with compromised foam.

In all likelihood, you just need to replace the helmet. Nearly all helmets, climbing and bicycle helmets included, need to be replaced once they become worn or older than 5 years. Ideally, you would track the age of each helmet. If you don't know how old one is and it looks well used, I wouldn't risk it.

Stick to gently wiping it with a soft dry cloth. Trying to clean a helmet with water or most any kind of soap only risks damaging the foam. If a helmet is so worn that a dry cloth doesn't restore it, it might be too worn. Consider how important your head is before try to cover it with compromised foam ( or compromised plastic, etc ).