If your backpack is washable canvass, you can completely empty it and run it through the wash.

If not, sprinkle baking soda on it and let it sit. You can then spray it lightly with white vinegar. Rub the foaming baking soda into the pocket.

If possible, sit it in the sun for a while where the light can hit the pocket to let the mixture dry.

Brush off any residue or remaining baking soda.