If the backpack is labeled as machine-washable, then wash it on the cold and gentle cycle with a very small amount of detergent. You may want to do the rinse cycle separately so that you can unbunch the bag after it has been washed to get out all of the detergent. Some insulated bags can also be washed in the machine, but it’s best to air dry them after. If an insulated backpack is not machine-washable, then fill a bowl with warm soapy water, dip a sponge or cloth in it, and scrub the inside and outside of the backpack. Then use a fresh dampened cloth to rinse and wipe off the residue. Finally, use a dry towel to wipe it dry and let it air dry overnight. To avoid washing it too often, you can just disinfect and deodorize it after every use. To do this, make a homemade solution of vinegar and water or hydrogen peroxide and water in equal parts and fill a spray bottle with it. Spray the bag inside and outside and then wipe it down with a dry cloth. The smell of vinegar will disappear once it dries.