Best way to fix tile
As long as the old mortar on the wall isn't crumbling you should be able to replace it without too much difficulty. Use a wire brush and get as much of the old mortar off the wall and the tile as you can, especially anything loose. For only one or two tiles I just use a good construction adhesive such as Liquid Nails for marble and tile. Use some spacers to keep your seams even, Support the tile in place while it dries. If the tile isn't too heavy you can use duct tape to hold it. Then just grout the seams. Good luck.