Steam it and stretch it over your knee.
A boiling teapot will give you enough steam. Hold it over the steam coming out the spout.
The felt should be hot to the touch but not boiling hot.
Steam mostly where you need to stretch it
You might have to pull fairly hard to stretch it enough, just stretch it a bit at a time though until it fits right.
The hot steam will let you reshape the crown and brim too if you need to.

edit.
Stetsons are way overpriced.
Smithbuilt or Baily are every bit as good.