Need help! How can i repair this?

Seems like water got in behind the frame and wall. Solving the problem would be the first step. Then you could use drywall patching compound to patch the area.

Like William suggests.....you have to find out WHERE the water came in....could have gotten in a distance away from the damage. Time to call in a professional carpenter to check around up there.

I agree with the others. Find and stop the water first. Then patch. Had a hard time seeing what the problem was in the picture. Call a carpenter to fix it correctly but find the water first. Spray outside first and have someone be inside watching. Spray cracks around the window or even grout if it is brick structure. Then you have to fix the grout area that is leaking, with new grout. Called tuck pointing and put new grout in the leaking area or cracks in the walls. For every crack you see on the outside, there are 100 you don't see. Stop the water first.

Looks like you just need to cut a sheet of drywall and mud it in.

Yes, stop the leaking first, then cut a new piece of wall board & totally replace that section. You may want to 'dress it up a bit' by added a nice stained wood strip around the inside of the window frame instead of having painted wallboard.