If your poster is not at the edge of the tube, turn it over and tap lightly to bring the paper to the edge.
Then, put two fingers in the tube and tighten the paper to make it smaller than the tube.
When wound tight enough, the poster will slide right out of the tube.
To return the poster to its tube, first roll it up. Then put two fingers inside the rolled-up poster again, and tighten until it is smaller than the tube and slides in easily.