John, to flatten rolled countertop laminate it only need to be unrolled in a warm place and allowed to relax. As you mentioned it has been rolled for a number of years, this process may take a few days. You can also reverse-roll the piece (roll it up inside-out from how it was) for a while to speed things along.
Of concern is that you also mention this is used laminate. I'm assuming you mean this is excess "scrap" material which was never actually glued and installed. Properly glued laminate would not be removeavble or reuseable. If this piece was previously glued down and has remnants of that glue still attached, I would not advise using it; it will leave voids and bumps in the new application.