If the new water heater is the same size as the old one and has connections in the same places, hooking up the water, gas, and flue will be simple. If you need to move any lines, the job will take longer.
The system should include a shutoff valve for the cold water line to the water heater (but not for the hot-water line) and the gas line. If not, install them when you install the new water heater.
In an area that experiences earthquakes, local codes may require that you chain or strap the water heater firmly to a wall.
Arrange for disposal of the old water heater. Your local waste hauler may have special requirements.
About a day for most installations
Groove-joint pliers, screwdriver, pipe wrenches, adjustable wrench, tools for cutting and joining pipe, two-wheel cart, torpedo level, hammer
Assembling supply pipe and gas pipe, maneuvering and leveling a large appliance, working with a flue
Clear a path for removing the old unit; prepare the household to be without hot water for a day; check local codes.
New water heater, shims, gas pipe or flexible connector, water pipe and/or flexible connector, heat trap nipples, pipe-thread tape, flue extension (optional), pressure-relief valve
Disconnect the water lines. If there is no flexible connector or union to disconnect, you may have to cut through a solid copper pipe. Use a tubing cutter to make a clean cut so you can connect the new line to it.
Carry or wheel in the new water heater. Position it so the water, gas, and flue connections are as easy to reach as possible. Use a level to check the unit for plumb and shim the legs if necessary.
Install heat-trap nipples to the water heater inlets. Install the blue nipple, its arrow pointing down, to the cold water inlet. Install the red nipple, its arrow pointing up, to the hot-water inlet. Tighten both with a pipe wrench. Run PVC from the pressure-relief valve to a floor drain.
If flexible water lines attach to the old water heater, simply unscrew the nuts with an adjustable wrench to disconnect. Check that the old fittings will work with the new; you may need to buy new lines or a male or female adapter. Check local codes.
Connect the pieces by driving three or four sheet metal screws into each joint. Test to make sure the flue draws and install a carbon monoxide detector in the room. If you have any doubts about the flue, have the gas company or a plumbing and heating contractor inspect it.