Are they just leaking a little where they come through the plastic?
If so you can pull the line up 3 or 4MM and they will seal again.
If not the easist way that I found to replace them is to get a new line approx. 2 or 3 inches too long, so you have something to hold on to.
Use a razor blade and trim the tip of the new line so that it effectivly becomes an arrow point and can be started through the hole.
Then with needle nose pliers you can grab the arrow tip and pull it the rest of the way through and trim to size.