Well, I'll take a stab at it. To remove use a hair dryer or CAREFULLY use a heat gun to soften the old sealer, and scrape with preferably plasitc scraper. I would use brakeclean to get the last bits and prep the surface.
To reapply, there are lots of opinions and options. I would use a polyurethane based product. A self leveling liquid is good for those flanged items like vents. Tape off the area and extend the sealer perhaps 3/4 - 1 inch onto the roof. Perhaps a dicor product (there are others).
For smaller gaps, like awning gutters, I'd use Vulcum or equivelent. (I believe there have been some name changes lately) You want UV resistant products on the roof, they last longer.
Many opinions on this, most not wrong, just a lot of preferred products and methods.
Use the search function and read up. There's a lot of info posted.