Go to an RV store and buy the product they sell for cleaning camper canvases.
There are several common household products that can be used to remove mildew. This is a page about vinegar for removing mildew from a tent.
Removing Tree Sap from a Tent?
Removing Mildew from Wood?
Vinegar for Removing Mildew from a Tent
Cleaning a Tent
Removing Applique Residue from Fabric?
Cleaning Yellowed Curtains