Step 1 - rip it out - remove all peeling, loose or damaged areas.
Step 2 - replace it correctly. Bed new tape in a thin layer of mud, let it dry completely, coat with progressively wider knives to blend, letting each thin coat dry completely, and lightly scraping off any high parts before the next coat fills the low parts. i.e. like doing any drywall repair properly.
Beware the common urge to try and make it perfect in one coat, or overworking ANY coat trying to "make it perfect" - perfection, such as it is, will be approached much faster by getting coats on, letting them dry, and getting more coats on.
If you are a video-type person, there are many out there, including ones from generally reputable sources.