Assuming you mean a backpack made of fabric or leather:
To deep clean your backpack, you'll need to empty it out completely and then spot treat any areas that are particularly dirty. To do this, mix up a solution of 1 part water and 1 part vinegar in a spray bottle. Then, use a soft cloth to scrub the affected areas before rinsing with clean water. Once you've spot treated the dirtiest areas, give your entire backpack a good once-over with the vinegar solution. Be sure to rinse it off afterwards with clean water. Finally, let your backpack air dry completely before putting anything back inside.