Good question and one that many hubbers find difficult to answer!
I myself do edit older hubs but only those I feel are worth refreshing. I must admit there are several on the waiting list for treatment but I'm too busy writing and researching new hubs so the older ones tend to get left behind!
Older hubs with low scores are said to bring down the other better scoring hubs. I'm not too sure about this theory so I do not delete older hubs, I radically edit them.
Right now I'm trying to concentrate on writing more new hubs to boost my overall numbers and earn a little bit more! It's a tough decision to make - I wish you well whatever you decide to do.