Empty your backpack and use an old toothbrush soaked in mild detergent to remove stains and dirt. Then, fill a basin or bathtub with warm water and 1 tablespoon of mild detergent. Soak the backpack for 30 minutes. Then, scrub all nooks and crannies of the backpack before rinsing it with clean water. Use a towel to remove excess water and let your backpack air dry.