You can tackle the issues that caused the urinating, such as a poorly kept litter box, later. But for now, your entire focus should be on getting the cat urine out of your backpack.
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Empty all contents of the backpack. Throw away papers soaked in urine. You may be able to salvage books if the majority of the urine is only on the cover. Wash any materials made of fabric in the washer and clean any hard items with soap and water.
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Sponge up as much of the cat urine as possible using wads of paper towels. Soak up the urine from the outside, as well as the inside, of the backpack.
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Pour straight white vinegar onto your backpack to help dilute the cat urine smell and clean the fabric. Blot up the vinegar with paper towels as you did the cat urine itself.
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Mix together a cleaning solution containing equal parts of hydrogen peroxide, white vinegar and baking soda in a bucket so that it has the consistency of a paste.
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Spread the cleaning solution to all areas of the backpack that were soaked in urine. Allow it to dry overnight.
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Use your vacuum cleaner hose attachment to remove the dried residue from your cleaning solution paste. Your backpack should no longer smell like cat urine.