Remove shingles around vent pipe. Remove old vent boot. Clean pipe surface. Install new vent boot over pipe. Attach to roof. Replace shingles around boot.
Leaks around the flashing or boot on a roof vent stack or pipe often can be remedied with a good tube of caulk. Make sure to use a caulk that willl remain felxible such as a butyl or elastomeric caulk. If you have to do an emergency repair on a wet roof, you can use plastic roof cement.