1. Find a ball of the right size, which can completely match the original shape of the hat.

2. Wet the collapsed part of the hat again with water to make it soft.

Put the hat on the ball and push it back to its original shape.

4. Wrap the hat in paper, try to keep it in this shape for a long time, and dry it in the shade.

5. Blow and iron at low temperature with a hair dryer.