Make sure you need a new boot may just need a bit of bitumastic caulk if it is leaking a bit at the duct contact as they commonly do as the rubber gets older.
Replace rubber boot on metal roof.
The best way for a do it yourselfer to repair a roof boot is to seal the existing boot and apply a new rain and repair pipe collar.
Step 1 clean pipe and boot.
I put in enough material that it squished out around the entire outside perimeter.
Now slide the boot down and make sure there is enough caulk to have a seamless circle all the way around.
You will need only a few materials and they will all be available at your local home improvement store.
Plumbing vent pipes that extend through the roof come with metal flashing or rubber boot type flashing to seal the penetrations.
If split then yes it needs replacing though a good wrap with waterproof duct tape or bitumastic caulk will keep it intact and dry until a roofer can fix it.
The slide the boot down the pipe and leave it about 3 above the roof.
You do not want to get the sealer between the rubber boot and on the plastic pipe.
Apply a liberal amount of silicone sealer to the old rubber boot.
Figuring that the rubber collar on the new flashing may split before it s time to strip and replace the roof shingles another 20 years hopefully i first seal the vent pipe to the metal of existing flashing with protectowrap s form flash 2 flashing tape.
Replace all rubber boots and replace metal penetration flashing.
Place the new boot in the hole of the pipe and cut the boot on the line that touches the rim.