To change the pinchroller, simply remove the "C" washer/clip using needlenose pliers, push the pin out, replace with new roller, and re-attach the clip. Make sure the dot is on the outside, as the rollers are coned. Also don't replace the outer rollers with the removable rollers, they are not the same.
@Artbot...the pinchrollers will wear out over time, and will skew the media. Ink on the rollers will shorten the life of the rollers as well.
With the machine on and the pinch rollers down, try pulling the material forward by hand. It should be very hard if not impossible to pull without damage. If this is the case, the problem is probably not the rollers. If it moves fairly easy, you can try changing the rollers first as GTS said. If this still doesn't help, it's probably the roller assembly as they do crack or the spring weakens. To change them is a major project requiring you remove the capping station. Good luck.
if the springs have weakenened, just take them off and cut about an 1/4-18" off and rebend the end and install. that said, you can go down the platen and "audit" the springs to see if you've got similar pressure all the way across.
i print on giant sheets of heavy aluminum and i've never stuck with the stock spring. as soon as in get a new printer i take off the springs and shorten them.