Get a bag of Synko Con-Fill.
https://www.rona.ca/en/synko-concret...-15kg-05285007
Scrape down a 4” square at the top corner. Make clean cuts with a blade to the existing drywall so you don’t have stray frayed paper poking out. Make sure anything within the square is a depression. Make sure a few inches of space around it are scraped smooth so you’re clear when you trowel in the fill.
Mix a small amount of the con-fill in a mud tray. I think it’s 1/2 powder, half water. You want it pancake mix consistency. Fill the 4” square and use a trowel to smooth it flush.
Clean the trowel and the mud tray quickly. It sets up fast and will harden on tools and be more trouble to clean.
Let it dry. Use a drywall sanding block to sand it smooth and flush to the adjacent area. Apply a second coat if necessary. Sand smooth. You can also use a dap polyfiller at the very end to fill any small remaining dips.
Evaluate how it looks. Then...
Option one: do that to whole area and then put on a new drywall primer before painting.
Option two: tear out the drywall and hire someone.