If you can see the mildew, brush as much as you can off the fabric. It is best to do this outside so the spoors are not in your home. Combine water and baking soda to create a paste, then smear the paste over the stained portion of the fabric. Leave it for 30 minutes, then rinse with water.
If the baking soda does not remove all the mildew, you can soak the affected portion of the fabric with lemon juice, then rub it with salt. Leave the salt on the fabric, and place it in the sunlight to dry, according to the North Carolina Cooperative Extension Service.
Lemon juice can have a bleaching effect on fabric, so test a small portion of the fabric somewhere inconspicuous to see if the color is damaged by the treatment. Mildew damages fibers in fabrics, so the stains it leaves behind are occasionally permanent, according to "2,001 Amazing Cleaning Secrets," by Jeff Bredenberg (2004).