The loose/peeling joints can be re-glued with seam glue. This should work well but could be difficult to keep the seams down while the glue dries. If this isn't working use a quick drying carpenters glue, you can also use something like super glue but either of these choices will never come off without causing damage to the wall.
To hide seams you will need some kind of spackle or patching compound. This could be applied over the seams (two coats), assuming they are flat to the wall, sanded then painted. If the seams are sticking up from the wall surface you will need to cut away this with a razor knife and then patch over.