You should always pay attention to the product description or the washing symbols on the side of the cover. They show you whether you may or may not wash them in the washing machine, for example - this is rather rare.
As a rule of thumb: wiping the cover with a damp cloth works without any problems in 99.9% of all cases.
Another option is to simply hose down the cover with a garden hose. This is also often recommended by the manufacturers of the cover. Important: The pressure should not be too high here, as this could possibly damage the fabric of the cover.