Before you can remove a tire from its rim, you have to let the air out of the tire. This can take several minutes if you do it by pressing on the valve stem, but it takes only a few seconds if you remove the valve stem entirely.
Removing the valve stem
First, remove the valve cap. As the cap is small and easily lost, put it in your pocket or some other safe place.
Using your valve stem removal tool or a slotted metal valve cap, turn the valve stem counterclockwise several turns until it comes out of the valve. You will feel and hear a rush of air if the tire is pressurized. As with your valve cap, put the valve stem in a safe place, as it is small and easy to lose.
After the tire is depressurized, replace the valve stem by starting it in a clockwise direction with your fingers then finishing with the valve stem removal tool or slotted metal valve cap. You will need it when you re-pressurize the tire and a small part like that is easily lost if not replaced immediately.