What are they made of?
If they are leather, I've had success just wearing them around the house.You can also try the hair dryer trick - put on a pair or two of socks, squeeze your feet into them, and aim the hair dryer at the areas that are too tight while flexing your toes and wiggling/shifting your feet around and putting your full weight on them. Keep your feet in there until the shoes cool back down, and then try them on after without socks. It's a bit uncomfortable, but it does work eventually.
If they are canvas or cloth, then dampen them, put them on and wear them around the house. As they dry, they conform to your feet and should loosen up.
If it's more than a half size too small, these tricks might not work as well.