how do you install bathtub faucet?

Which kind - a spout on the wall over the tub, or taps mounted on the tub itself? If it's the wall spout, that's the easy one. They're either slip-on, with a small grub screw underneath (usually an allen key), or they thread onto the pipe, in which case the whole spout turns off anticlockwise. Measure how far the pipe sticks out of the wall before buying the new one, because slip-on spouts need a set length of pipe to seal on. PTFE tape on the threads for a threaded one. Anything involving the valve behind the wall is a different job altogether.