Set up your drill with the grout-removal bit. Hold the flat edge of the disk above the grout line that's next to one of the tiles you want to remove.
Engage the drill and get it to full speed. Lower the edge of the disk into the grout, grinding and breaking the cement. Sink the disk to the depth of the tile (usually about 1/4 inch), the begin moving it forward along the line.
Run the edge of the disk along the grout line next to the tile, and continue alongside any adjacent tiles that you're taking out. Move the bit to other grout lines as needed, running it along all four sides of each tile that's coming out.
Press the tip of your chisel into one of the opening grout lines, against the base of the tile, and tap at the end of the chisel with your hammer. Repeat the chiseling process around the whole edge of the tile, on all sides, until it comes out.
Scrape the wall with a razor scraper to remove any residual mortar. The area is now ready for re-tiling.