I ran into a similar situation with my daughter-in-law's iPad mini that had a bent frame. Frankly the only safe way to straighten it is to take off the screen; otherwise I guarantee you're going to break it trying to fix the frame.
In my case I researched suppliers and instructions on how to replace the entire housing before I opened it up; fortunately I was able to bend it back into shape so I didn't end up needing a new frame, but that's something you might consider. The iFixit teardown would basically get you most of the way there, but it doesn't continue to the point where everything has been removed from the housing so that might be something you would have to work out yourself.
[guide|156238|iPad Air (5th Generation) Teardown - iFixit|new_window=true]
Should you decide to go for it, be sure to take lots of pictures and when you're done you can use them to create your own guide to the housing replacement! It's easy, fun and will earn the respect of the iFixit community.
Good luck and let us know how it all turns out; each persons experience, whether successful or not, helps us all know what to do and what not to do.