Apply joint compound to either side of the corner, covering the bead patch to clean rough edges and cover any seams, feathering the perimeters. Allow dry and sand easy. Now you may need a piece of plasterboard to fill the opening. Ideally this must be thinner than The present plasterboard. https://cruxbookmarks.com/story18194554/how-stainless-steel-fixings-supplier-can-save-you-time-stress-and-money