Repairing water damage to ceiling drywall where ceiling meets wall
You'll want to remove a section for repair and inspection. Keep in mind, the hard part of wall finishing is the plastering. Also the worst part of the water damage is going to be in the least visible area, in behind the wall. Cut out a section of the damaged wall large enough to inspect behind. Mold is a concern but once it's dry it tends to die. Anti mold spray is available for exactly this purpose.

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