best way to replace a faucet in the bathtub?

If you mean the spout, feel underneath where it meets the wall. A small hole with a set screw means it's a slip-on: loosen the screw with an allen key and pull it straight off. No screw means it's threaded, so it just unscrews anticlockwise. Wrap it in a cloth if you need grips. Buy the new one to match the type and check the length of the pipe sticking out of the wall, because slip-on spouts only seal if the pipe sits in them the right depth. Plumber's tape on threads, a thin bead of silicone along the top edge against the tile. If you meant the handle or valve that's leaking, that's a cartridge swap, which is a different job. Which one is it?