Brushing the canvas with a scrub brush will remove mildew that has dried. Excess debris can be washed off with a mixture of mild detergent and water.
A mixture of 50/50 rubbing alcohol and water may also remove mildew, as indicated by Mrs. Clean USA. Submerging a towel into the solution and using it to wipe the affected area can clean away mildew. The awning should be rinsed with water after cleaning.
Using bleach on your awning can ruin the fabric. Chemicals contained in bleach can cause a reaction such as staining on your fabric.
Using a mixture of 50/50 white vinegar and water can help prevent mildew growth. The solution can be applied in a spray bottle after cleaning your awning.