How to Clean Outdoor Cushions

Brush off loose dirt.
Spray on a cleaning solution of water and mild soap.
Use a soft bristle brush to clean.
Allow cleaning solution to soak into the fabric.
Rinse thoroughly until all soap residue is removed.
Air dry.

Source: Sunbrella

If the cushions zip off, wash in machine in cold water then let them air dry and put back on the insert while still damp.