Boil a tea kettle or a pot of water on your stove to create steam.
Hold the hat in the steam top-down so that you do not get the hat band wet. Be careful not to burn your hands on the steam. Hold the hat in the steam for 10 seconds and then remove it.
Firmly press the hat down over your head. The steam will make the felt pliable, and it will reshape to fit your head. Wear the hat until it dries.
Repeat the steaming process as often as you need to stretch the hat to fit your head. You may need to perform the process a few times if the hat is very tight on your head.