You'll need to remove any loose tape and drywall compound, then re-tape the corners. That involves laying in a thin bed of drywall compound, laying the paper tape into it, and using a taping knife (like a big spatula) to smooth the tape into the compound and squeeze the excess compound out from under the tape. It will look like hammered shit at this point, don't panic. You let that dry, then apply a thin layer of compound over top, smoothing and feathering it out to blend into the wall. Let that dry, then sand it down smooth and apply another coat, feathering it out further this time. The goal is a seamless transition, so you may need several thin coats, sanding in between, to get it to look good.

It's a specialized skill, and it takes practice, but it's not very complicated. Just takes patience and a strong sanding arm. Make sure you get it all as smooth as you can before priming and painting - it's much harder to sand out any imperfections once there's paint on the wall.

The outside corners might not have paper tape, they might have metal or plastic corner bead that's attached with nails or screws. If that's the case, and the corner bead is solid and not loose, you should be able to just knock off any loose drywall compound, give it a rough sanding, and smooth over a couple of new layers of compound.

For the inside corners, after the initial bedding in of the tape, you'll want to work the walls one at a time. Do one side, let it dry, then do the other side. Let that dry, then sand both sides together. Repeat. Unless you have a lot of practice, it's very easy to screw up the side you just finished if you try to do the opposite side while the compound is wet.