Without Quick Release Removal
Shift the bike's gears to the smallest cog. Keep the bike upright.
Open the brake by lifting it up and away from the tire. The brake pads will move away from the rim, and this will allow the tire to pass through freely.
Stand on the side of your bike that does not have the chain and flip the quick-release skewer to the "open" position. The skewer is what attaches the wheels to the bike.
Grasp the bike's top tube with your left hand and lift the wheel off the ground; the wheel will fall out of the dropout.
Quick Release Removal
Open the brakes so that the pads do not touch the wheel rim. Open the quick-release lever.
Hold down the quick-release lever while loosening the nut located on its opposite end if you're removing a front tire.
Lift the wheel out of the dropout when the nut is turned enough so that both sides of the quick-release skewer clear the safety tabs on the fork, which connects the frame to the wheel and handlebars.