You probably know how to change a tire. But tires can be tight these days. And who wants to reach for an extra tool when you've already got two perfectly good ones at the ends of your wrists.
The "floor" between the rim's sidewalls, is never flat. It’s always lower in the middle, sometimes by a lot. The natural shape of a tire will want to spread the beads outward onto the slightly taller banks along the rim’s outside edge. This will make a tight-fitting tire fit even tighter. If you get to the last 12 or so inches, and the bead doesn't want to pop over, try pushing the bead you’ve already mounted further into the center of the rim. The circumference here is often an inch less than up on the edge, essentially giving you an inch more slack to work with.
As the tire loosens its grip, keep pressure on the stubborn section as you work your way around the rest of the tire. Once it's all in, you should have no problem popping the last bit of bead over with your thumbs.