The top and bottom of your cover should be cleaned periodically. Before we begin remove the cover so cleaning agents and dirt don't enter the line. Find a non-abrasive surface such as grass to set your cover down. With a hose wash off any dirt, leaves or other debris.
Scrub any stubborn spots with warm water mixed with a mild soap and a sponge or soft cloth. Once the top is clean rinse thoroughly flip the cover over and repeat this process on the bottom. If stains have developed that don't respond to soap and water then we'll need something a little... stronger.
Mix 1 cup bleach with 1 gallon of water and apply to the stain. Don't forget to thoroughly rinse afterwards. Also, keep in mind the best way for this side to stay in tip-top shape is to keep your water clean and chemicals balanced. For help with best practices check out our how-to video that covers routine water maintenance.
Now, using a soft absorbent towel or chamois dry the bottom then flip the cover and dry the top. The final step is to condition and protect the vinyl you can use a product made for hot tub covers or any marine grade vinyl; however, you do not want to use petroleum, silicon or citrus based products. Follow the instructions of the product you're using, typically you just rub it in.
Also, never store anything on the cover and don't allow twigs leaves or other debris to collect and sit for too long. Replace the cover and you're done.
For European countries, please refer to this Water Chemistry Guide for appropriate chemical levels per BS EN 17125:2018.