The baseplate is supposed to remove going forward. It's not unknown for dropped magazine floorplates to dent from being dropped and be hard to remove.

Depress the lock pin in the middle of the floorplate and move the floorplate forward. When it stops moving, light taps with a tack hammer on the corners will move it forward. You can do some refitting after you get the guts out of the magazine.

Check the parts diagrams. Some of the magazines required a spring change too in order to go up one round.