Oh man, the infamous duo: those damn rivets and non-caring postal workers claim yet another victim. The front can be done quite easily, but the back... Ouch!

You'd have to strip everything (and I mean everything, the gfx card, CPU(s), drives, fans, PSU, Mobo) to dismantle this element and try to fix it, but I wouldn't do this alone. It's not just "grab a rubber mallet and hit it where it sticks out", too much metal to reshape. And then - the riveting (or rather welding, or maybe if you have a strong two-component epoxy glue - with a bit of luck - glueing) everything in place... Wouldn't it be better to get a new case?