You can cover the lip of the damaged boot in mastic or silicone caulk.
Replacing roof vent pipe boot.
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.
There are a number of different methods for repairing a roof boot.
Easy installation and no tools required.
Slides over the existing roof vent boot preventing future leaks.
I purchased a plastic flashing assembly from the local home improvement store.
Designed for all standard roof pitches 3 12 to 12 12.
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.
You can also replace the entire flashing around the pipe.
Now it is time to work on the roof.
Obtain another rubber seal.
Works for any size pipe.
However replacing the entire flashing can be quite involved and may require the application of new shingles.
Use common sense safety precautions and make sure you can.