This post will cover wool and felt blocked hats. It's easy to resize them a bit, but sometimes a hat is just hopelessly too small or large. You'll want to determine that before you try any of the below steps and see if you can possibly exchange the hat for one in the proper size.

For a hat that's too large, try a strip of felt or weatherstripping inside the band. You may be able to get down a size smaller with this method.

If a hat is too small, try a hat stretcher (you can buy them online or at some western wear stores). Remove the ribbon inside the crown if you can. Steam is also your friend. Use caution with steam. It's hot! If you're not comfortable with this step, seek professional assistance. (At this point you may say "duh," but please be careful. burns are no fun.) You can steam the inside of the hat using a garment steamer or tea kettle and from there stretch the hat. While it's still damp (but not hot) you can place it over your head and wear it while it dries. Use caution! Of course, if the weather is cold, you'll want to stay inside where it's warm. If you think you're coming down with a cold, don't do this until you feel better.

Wool and felt will expand and shrink, but not by a lot. And don't get the hat soaking wet as you may lose some of the sizing. 

I live in Texas, and we do have western wear stores that will steam and shape hats. If you have one in your area, that's a good place to start for hat care supplies and professional assistance. 

Enjoy your new hat!