STEP 1: REMOVE THE TIRE
Take your wheel off. Slip the flat end of a tire lever under the bead; hook the other end onto an adjacent spoke. Insert a second lever under the bead and slide it between the tire and rim until one side is completely off. Remove the other side and pull out the damaged inner tube.
STEP 2: INSPECT FOR DAMAGE
Run your fingers along the inside of the tire to feel for and clear anything that may have caused the flat—a thorn, a piece of glass, a nail. Two side-by-side holes (like a snakebite) usually indicate a pinch flat, which is generally a result of underinflation.
STEP 3: INSTALL A NEW TUBE
Seat one side of the tire onto the rim, then tuck a new, slightly inflated tube inside—starting at the valve hole and working your way around. Using the palms of your hands, roll the other side of the tire onto the rim, making sure the tube isn't getting pinched as you go.
STEP 4: INFLATE CAREFULLY
With your pump head attached to your valve stem (there are two types: Schrader and presta, the latter more common on road tubes), inflate the tube to half your desired tire pressure. Make sure the tire is seated properly on the rim, then pump until you reach the desired psi.