Comparing

[How to uncurl/flatten a poster]
1.Remove your poster from the tube by tipping it until it slides partly out.
2.Carefully reroll the poster around the outside of the tube, the opposite way to which it has previously been rolled.
3.Secure with a sticker or rubber band and leave this for an hour 

You can use also a second method, recommended for 21x30 poster: 

1.Roll out the poster on a clean, flat and hard surface like a table 
2.Place something heavy on each corner and in the middle to weigh it down – books work particularly well
3.Leave for 24 hours