|Well first I would need to know if it is an electric or gas water heater.Then I need to know what brand it is. for example...
Ruud, Rheem, Whirlpool, Kenmore, etc.
If electric then shut off the breaker to the heater. Shut off the water to the heater. You will need some copper pipe soldering experience to do this install. Cut the inlet and outlet pipes with a pipe cutter. Remove the electrical wiring from the heater too. Drain the heater of all water, there should be a drain right at the floor that looks like a outside spigot. Position the new heater and install in reverse. Electrical last. Check for leaks, and let it fill. Get the air out by opening up a faucet somewhere until the air is gone. Turn on the electrical power.
If it is gas, the same applies except for the electrical. There is none. Turn off the gas on the pipe that goes to the heater. There should be a union between the shutoff and the heater where you can undo the gas piping. If the new heater is the exact same you can just swap parts. Use a gas approved pipe dope compound on the gas fittings. Do not use a flexible pipe unless allowed in your area. Now all you would need to do (after the water pipes are installed and the tank is filled or course) is light the pilot!