Vinegar is a natural disinfectant and can be used to kill mold. Just mix vinegar with water in a ratio of one part vinegar to three parts water. Soak the backpack in this mixture for about 30 minutes. Then, rinse it well with warm water. You can also use tea tree oil to get rid of mold. Add a few drops of tea tree oil to a cup of water. Soak the backpack in this mixture for about 30 minutes. Rinse it well with warm water.