All of the above is good advice, but before you put your backpack in the washer, remember to clear it out of anything that might get damaged in the washer.
Start with the pocket in the front. This is pretty quick to unzip...and it is shallow, so you should be able to clear it out of a USB cable, a proprietary USB cable that doesn't fit any device you have, a few pencils, a few leaky pens (better go wash your hands), 63 cents of US currency and a few Canadian pennies from 1982, and a packet of parmesan cheese from a restaurant you visited a few months ago, and a...
Okay, maybe we should do this biggest to smallest. This thing feels pretty heavy! Lets go for the big pocket first. You pull out a paperback textbook that costs more than the mortgage payment on your parent's first home, and that you hate, but not as much as the teacher does. You pull out a notebook that has some scribbling on the first few pages, some scribbling on the back few pages, and is mostly just empty, but rumpled, paper, but that still weighs you down. There is a paper bag, complete with napkins, that a donut came in, and a receipt for the donut, covered with maple sugar. You lick your fingers, and keep digging. There is a pair of socks that you wadded up and put in the bag a few weeks ago, when there was an expected rainstorm. You are, after all, a very organized person. There is some maple frosting on the socks, but you have too much dignity to taste it, despite its rich maply smell. You find a licensed sci-fi/fantasy property novel that you have been carrying around to read when stuck waiting in places, and that you are somehow 120 pages into after a year of it being in your pack. An empty soda bottle. Okay, that is all the big stuff. Some more coins here. When was the last time you were in Mexico? Why are there all these pesos?
Okay, lets go to the middle pocket. Ibuprofen, naproxen and acetaminophen, which you haven't even used in a while, but maybe you will need them. The horror of getting stuck with a terrible headache somewhere. Some little loperamide tables, crushed up in their blister packs, for similar reasons, because you never know when, but you know by the time you got the fragments out after tearing that foil with your teeth, they would fall on the ground as a fine powder. Some more pens and pencils, this time without any leaking, at least, but all the pencils have broken points and the pens are dry. A tiny pamphlet for your community college's bowling team from 2003 (okay, got to point out: of course this hasn't been in the backpack since 2003, only for the past few months, because you wanted to show a friend how lol random it was, but then it got crushed down there, but the graphics are still glossy, at least by the standards of the turn of the century). Some taco bell hot sauce packets---maybe you could use them to clean those Canadian pennies, like this was a Sierra text adventure game. More coins. Wait, tokens, for a beachfront arcade and your laundry room in Santiago. You put them and the Pokemon dice in a pile next to the backpack. There are also cracker crumbs. A washcloth because you are a hoopy frood, and a moist towlette you picked up on a hotel or train and thought you might need...
Okay, all cleared out. Wait, you didn't even finish with the small pocket, and now you look out the window and you see the sun is already touching the trees on the horizon. At this time of year?, well, wait, it is November, easy to forget when you are in a tropical country? Wait, tropical country? No, you can hear the hum of baseboard heaters in your standard American apartment, why did you think "tropical country", does that have anything to do with the assortment of foreign coins, and also the crumpled Alaska Airline boarding pass in the inside zipper pocket of the medium pocket? Did you forget something, you wonder, as you untangle some headphones for the Tracfone and open a Nature Valley Granola Bar that instantly crumbles to flakes, even more than usual. What places has this thing seen, you wonder, as you congratulate yourself that your backpack is almost ready for washing.