It’s been a long time since I used one of those Edges. I think that those connected on to the mount by sliding sideways. There was a tab on the mount that would click and lock the Edge into place, and to remove the Edge, you pushed the tab down and slid the Edge off of the mount. If that’s the case, you need to figure out where the tab on the mount is, and insert something like a thin standard screwdriver and push the tab back, and then slide the mount and Edge apart from each other.
ETA: If a screwdriver doesn’t work, look around your house for something. A stout piece of wire, or a wire hanger might be able to be bent in the right shape to get in there. I have an assortment of old dental tools that have come in very handy over the years for things like this. If you can’t get to the tab, you might be able to use beak-shaped wire cutters to carefully break the mount apart until you can free it.
.
