Locate the on and off switch for your spa filter pump. Turn the filter to "off" position. Look for a round valve either by the pump or on the filter. This valve will have a handle that lifts up and down. Most of these handles can be pressed down and turned to the backwash position indicated on the valve.
Turn on the filter pump. The water flow will be going in the opposite direction into the filter cleaning out all accumulated debris. Let this run for at least one minute. You will see that the water at the beginning will be cloudy and then will turn clear. When the water turns clear turn filter pump off.
Go back to the valve and press the handle into the down position. Move it to the SPA position or back to the original position before you started cleaning the filter.
Turn on the filter pump. This should complete the procedure. Now that you backwashed the spa you may need to fill the spa with water back to your desired level. You should backwash your spa at least once per month.