Probably best not to do anything unless you have much experience. You will need to take the pack apart and test the voltage of each cell. They should all be the same, if one or more is down on voltage then probably duff. The problem then comes when you have to replace it with a cell that is matched to the existing ones.

Probably less time consuming to get a new pack. I have lots of cells at home which all need testing and sorting, just dont have the time.