Oh geez.. You're not going to like this
If it's anything like my Thomas (and it probably is), the top of the sheet metal is bent 90° and is pinched between the piece you see directly below the window and a piece of angle iron in the wall. Perhaps you could yank it out with great force, but since I didn't plan on reusing the sheet metal (at least not for that purpose) I just cut it out with a grinder and a cut off wheel.