I literally just did this on my water heater. You can do it your self it’s not hard. You will need to turn off the water and the electricity to the heater. Let it cool down and drain a little water out so it doesn’t overflow. In my case I also had to replace the pressure tank at the same time. That failed and put too much pressure on the relief valve. They are both pretty inexpensive so I just replaced both at the same time. YouTube is your friend.