Take it to a seamstress and have it re-sized. If it’s a cap and not a beanie it won’t shrink evenly and will be really ugly, you could also fold up a hankerchief or bandanna and place it inside the headband, that will make it fit better.
The inside is fleeced lined so I wouldn't be able to add a handkerchief. I suppose what I could do is sew something in. It's a bummer I like the color. It might cost more than the hats worth to re-size it. Hmmm