Use a wrench (usually alan wrench) to remove the bolts (axles) that pass through the bearing hubs. Once the bolt is removed, the wheel assembly should come free and simply slide out of the boot. Flip the wheel, and replace in the boot. Reassemble, making sure to make the bolts tight, but not too tight. The wheels shouldn't be squeeze by the wheel frame at all. While you have them out, consider oiling the bearings and/or replacing them. Do not use WD-40. It's not oil.