Vaclav, from Edmonton, writes: "Years ago the tile bathtub-surround was cut out and replaced with new drywall and tile. The upper edge of the drywall tape, which is not covered with the top row of tiles, is peeling off. How can I fix it without removing it?"
Open the crack behind the tape just enough to get a toothpick into the crack. Use the toothpick to force some "Outdoor Wood Glue" into the crack. You want to use a very liquid but waterproof glue. Push a bit on the crack to work the glue down into the crack and onto/into the old joint compound. You see, joint compound will become soft again with any liquid coming into contact with it so the glue will not only penetrate but mix with the compound. Wipe off the excess.
When it dries this will be a waterproofed area that can be touched up with a dab of paint. The best way to hold it all tight until it dries is to wrap a block of wood in wax paper and then jam it into place with a stick to the other side of the room. If you can't do that, then use a wide masking tape to hold it in place. To avoid having the masking tape glued to the wall, put a small strip of wax paper on the sticky side of the masking tape right where the glue joint will be.
There is a video on Tape Delamination in the Learning Curve Tab above.