Here is what I've done with mine in the past.
Take it to a coin laundry (because of the industrial size machines) and run it through a cycle with a bottle of Nikwax Tech Wash, or similar soar safe for waterproof material, available at somewhere like
www.rei.com.
hang it out to dry.
When dry, lay it out upside down and give it a liberal spraying with something like 303's fabric guard/waterproofer. I buy the gallon jug and use a clean pump-up garden sprayer.