Are you planning on just replacing the bottom without removing the glass?
I would recommend to separate the inner and Outer frame to prevent rust comming through later.
Just continue from the side to pry it open.
If you use a vise grip like this one on the inside corner,
you can keep the material flat while bending it 90 degr open.
You must open the sides of the top lip too, just until the tab left and right.
Looking at your bottom you probably have to replace...