To replace a hot water heater, you need to follow these steps:

1. Purchase a new water heater that matches the specifications of your old one.
2. Turn off the gas or electricity and the water supply to the old water heater unit.
3. Drain the old water heater tank and disconnect the water and gas piping.
4. Remove the old water heater and bring in the new one.
5. Install the new water heater by connecting the water and gas piping and the heating element.
6. Refill the water heater tank and turn on the power and water supply.
7. Test the new water heater for proper functioning and leaks.

Here is a more detailed step-by-step guide on how to replace a hot water heater .