YouTube is your friend. You can buy a spray chemical which softens the leather. Tried it on the wife's cowboy boots, doesn't work!
1. Mark the areas on the outside of the boot with chalk where you think that they are tight and put on a thin pair of socks, borrow some sock tights from the woman in your life if you can.
2. Find a ball hammer or hard round object that you can use in your hand to get to the inside of the boot, we are going to use this to push and stretch the leather at the tight spots.
3. Get a deep pan half filled with water, build a spout smaller than the diameter of your boot using aluminium foil and sit it on top of the saucepan so that when the water boils, the steam comes out of the spout. Get a pair of tongs and hold the boot upside down over the spout so the steam gets into the boot.
4. Get some steam inside the boot, get it nice and hot and start stretching the boot from the inside and when it is cool enough to work with your hand tool.
5. Wear the boots indoors for hours at a time before you go out in them.
Alternatively send them back and get a size bigger or go and try on some other boots