I first advise that if you are not comfortable on a roof or if the pitch of the roof ( the steepness) is to great you might want to hire a professional. that being said this job can be done by most anyone with the proper tools, patience, and a dry day.
Step 1: Assessing the Job and Planning for What You Will Need
Step 2: Removal
2. Then i proceeded to un crimp the top cap, these metal pipes are a pain and are meant to stay on so some damage will be done to the cap while un crimping.
3. once the top cap is un crimped you can work on the skirt these are on TIGHT so it will take a little effort to take them off. i first cut the sealant, then grab bot sides of the skirt and worm is to the top took about 5 min to get it off.
4. Now you can remove the boot should come off with a little bit of ease depending on how much sealant they used.
5. once the boot is off start to remove the debris and old sealant from around the boot.
Step 3: Installation
2. The slide the boot down the pipe and leave it about 3" above the roof. and begging to caulk around where the boot will go on the roof.
3. Now slide the boot down and make sure there is enough caulk to have a seamless circle all the way around. (never too much caulking so use as much as you want )
4. Start screwing the boot down about every 1 1/2" or so until it is completely filled in.
5. now caulk around where the boot and the pipe meet to make it waterproof.
6. once you have done this look around where the old boot was and look for screw holes and place a nice pile of caulk on each one, then place a screw in each hole. to fill the gap.
7. Now reinstall the Skirt just over top of the boot and caulk the gap around the top.
8. Then Put the top cap back on and if necessary place a screw to hold it on.
9. Clean up and don't fall off the roof, your done
Side note: this is the pretty much the rule of thumb for and pipe on a roof with exceptions to electrical pipes on those you will need a zipper boot. those are a different story. use caution of the roof use boots with good grip.