First, completely empty your backpack, open all the pockets and compartments, and shake out debris, sand, and dust from them. If the degree of contamination is light, you can wipe the inside of the pack with a damp cloth and remove dirt and stains from the outside with a toothbrush or sponge soaked in soapy water. If your bag is very dirty, it should be washed thoroughly. You can do it by hand using mild soap, or use a washing machine if a manufacturer has written about it in the instructions. The pack should be turned upside down and hung in a well-ventilated area until dry.