Deflate the tire completely
Remove the valve cap
Remove the valve core using a valve core tool
Break the tire bead away from the rim
Push the valve stem inward through the rim hole
If the stem is rubber, cut or pry it out from the rim hole
If the stem is metal, remove the retaining nut and pull the stem out
Clean the valve hole before installing a new stem
Install the replacement valve stem
Reinstall the valve core
Inflate the tire to the recommended pressure
Check for leaks around the valve stem