Replace a roof vent by first pulling up the shingles that cover the base of the vent. This metal base is called the vent flashing and is a notorious spot for roof leaks, explains The Family Handyman.
To replace the vent, first use a crowbar to pry the vent flashing from the surface of the roof. Remove the old vent cap from the top of the vent pipe. Once the vent flashing has been released, lift it carefully over the main section of the vent pipe. This should expose the old caulking that adheres the vent to the surface of the roof. Scrape away the old caulking. Next, pry the old vent up and off the pipe. Replace the vent cap with the new vent, and caulk it against the roof. This should be done by lifting the vent about 2 inches and spreading caulking around the surface of the roof where the bottom of the vent is to be placed. Slowly settle the vent onto the caulking, and press firmly to seat it. Slide the new vent flashing over the vent pipe, and seat it against the surface of the roof. Secure the flashing with three or four 1-1/4 inch roofing nails. Replace the shingles to cover the new vent flashing, and affix the new vent cap on top of the vent pipe to finish the vent installation.