You may have to cut into it from both places. I would start at the top.
You should be able to remove the tiles fairly easily using either a grout rake or a dremel with a tile bit.
Do you know what is under the tiles? Cement board? Wood? etc..
Another approach is to use a right-angle grinder with a diamond wheel to cut into it. That would be the best way to cut into the cement board, as well.