On the main jetblue webpage, you should see a link on the left to "view/change your itinerary" - put in your confirmation number, last name, and the cities your flight originates and ends in. You should go to a page that shows your seats and has a button that says "change seats" -- then the diagram of the plane with available seats comes up and you can pick your seats.
If this doesn't work, I would just call jetblue and ask them to change your seats over the phone -- I'm sure it wouldn't be a problem.