YOu need to remove enough so that when you thinset a new tile down, it can be made level with the adjacent tile. Thinset is probably your best bet, but buying a whole bag for the use of a cup full is a pain. Once the bag is opened, it doesn't or at least shouldn't have much shelf-life left. Check out www.johnbridge.com for help on tiling issues. A hammer and chisel should get enough off fairly quickly to let you reset the tile. Remove the grout around it before you try to remove the tile, or you'll be replacing more than that one.