Stuck the whole backpack in the washing machine. I had a mesh bag meant for securing stuff like shoes and whatnot and wrapped my backpack in it. Mind you, my backpack is mostly made out of fabric.

I wouldn’t advise washing it like I did if it’s made of leather, etc.

Careful of any loose straps if you're just putting the whole thing in the washer. I had one my favorite backpacks get eaten/torn by the washing machine rotors because one of the straps somehow got stuck.

Would recommend air-drying because some of the stuffing materials they use for padding like on the arm straps can shrink and become useless in the drier.