Hose it down on a nice sunny day, let it dry and do it all again, and if possible do it a third time. Let it dry fully before packing away.
There are some 'waterproof' sprays available but should not be necessary if canvas is seasoned properly. Spray may be used on seams if they do not seal up fully. Canvas will leak if anything is touching it while it is wet so keep stuff away from walls when rain is about. The wetting down is to allow the canvas to shrink tight.