You may be talking about a valve between the pump and the fresh water tank. You should see a hose connection outside labeled antifreeze. You might see another valve there too. Hook up a short hose to drop in a bucket of antifreeze, close the valve between the pump and tank, and open the one at the hose connection if there is one, turn on the pump and open each faucet until antifreeze comes out. Leave the valves all winter.
If you're talking about a valve at the hot water heater, that's there so you can drain the water heater and avoid filling it with anti-freeze. You probably have two or three valves there and you can google Hot water heater by-pass valves, match your configuration and set the valves for by-pass before you take the steps above to fill the lines with antifreeze.
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