Even with regular treatment of your hot tub's water, it's important to drain the tub regularly, clean it and refill it with fresh water. The drain plug of your hot tub is designed for just this purpose. It's not hard to use the drain plug, and employing the plug to empty out the water is the first part of cleaning and refilling.
Turn off the power to the hot tub before draining the water.
Grasp the cap on the drain plug and pull outward to extend the drainage tube. Remove the cap on the drain plug by turning it counterclockwise with your fingers.
Screw a length of garden hose onto the end of the drain plug. Uncoil the hose and place the end where you want the water to drain to--in all likelihood, below the level of the drain plug so the water drains by gravity.
Push the end of the drain tube back in toward the hot tub. The water will begin draining through the hose.
Disconnect the hose when the water has drained out and replace the cap. Push the drain tube back into the hot tub.