You probably have two things that you can use...they're probably sitting on the keyboard and mouse right now
Seriously, it is a lot of work. You can try a hair dryer, depends on how old the glue is. How big is the section you want to replace? Is it just a cut or a couple of square feet? A thin, flat bar and some elbow grease is probably your best route.
Much like the other guy said, if the tile is pretty old, nothing but some hard work is going to peel it up. Home depot sells a tool for about thirty bucks that's supposed to be for this, but they just don't work. Your best bet is to use a hair dryer or clothing iron to try to loosen the glue, then use a good strong putty knife or trowel to get underneath the tile and peel it up as best as you can. When you have as much as you can possibly get up, you can use a belt sander to remove the rest. (If you are working on a wooden foundation. )If you are working on concrete, you might try a little paint remover on any leftover glue. I have never tried it, but I can't imagine it hurting anything