Utilize joint compound to both sides of the corner, masking the bead patch to easy rough edges and canopy any seams, feathering the edges. Let dry and sand easy. Now you will need a bit of plasterboard to fill the opening. Preferably this should be thinner than the current plasterboard. https://wearethelist.com/story19981962/the-basic-principles-of-plaster-board-fixings