This hack will fix lippage as long as they're between 1/32 inch and 1/16 inch, depending on the grout line thickness, and there aren't unleveled tiles on most of the floor.
The easier method is to take a fine-grit sandpaper and slowly sand the edge of the tile to make it even with the one next to it, then polish it and seal it.
If the lippage is significant, add grout to the tile's seam as you would during the first install. Then, add more grout, dragging the float upward toward the higher tile.
Drag the sponge longways down the grout line to create an angle sloping down to the lower tile, making it even. Let the grout dry, then clean the seams and remove any excess.