Cushion covers can be washed very easily. Just remove from the cushion insert, turn inside out and close the zipper. Wash by hand with a mild detergent or on a delicate/silk cycle in your washing machine. Never wash in water warmer than 30 degrees.
Do not dry in a dryer, do not dryclean.
If your cushions have been out in the rain and have become very wet, take the covers off and leave the foam inserts in the sun to dry out properly.