How would you fix this?

Looking closer at it thru your pictures,, That rounded piece,, I call that the top,, Is hung up on the top of the wood,, Use a flat bladed (Big) screw driver and pry it away a little so you can push or pull it back down,, You might have luck with a good (big) set of Vice grips,, get a purchase point on the lip and yank it down into position,, Then screw it or staple it back to the wood strips, If you have some gaps at the side wall seam,, Hvac metal tape or make or use 90 degree L brackets and screw them in to hold and tape it up!
Let us know what you do,, post more picts!