Remove the map from the cylinder in which it came, or remove the rubber band holding it together.
Hold the map at outer edge and carefully roll it in the opposite direction from which it was rolled before being stored. Make sure you don't damage any of the corners or edges.
Place the rubber band back onto the map to secure it in place, or put the newly rolled map back into its cylinder. Let the map sit for a few hours.
Pull the map out of the cylinder or remove the rubber band and unroll the map.
Lay the map on a flat surface, securing the corners with books if the map hasn't completely flattened yet. Leave the books on the corners until the map no longer curls.