You carefully remove the foam from the shell and put it back when you are done.
How old is the helmet? If it is fairly new you should contact Bell and ask them if they can help you with this issue. If it is more than 3 or 4 years old you would probably be better off to replace it.
Yes as above....these helmet's EPS (the shock- absorbent bit) degrades even on the shelf....
Thats part of the reason you often see a $800 helmet marked down to $200.....bout 5years life from new.
Old helmets do make a good garage decoration tho....same as any that did their job in protecting you...they (the later) dont deserve to be binned