How do I change the membership end date in my membership database?

I have imported 86 existing members into my new website with membership plugin and they have all been given a membership end date of one year from today. In fact they all have different membership end dates. How do I change the date - I'm willing to do it manually. This is really important and a 'show stopper' if I can't change the dates. I see from the forums that other users have had the same problem for a year or so now.
Gill