To get your pack all cleaned up, you only need a few things: a mild, non-fragranced soap, a basin to place the pack in, a sponge, and a soft brush like an old toothbrush.
First, unzip all the pockets and shake out the loose debris from your pack. If it has any detachable parts, like the straps, remove them and set aside to wash separately. From here, submerge the pack in a deep basin with lukewarm water, but don't allow it to soak. This could be in a plastic bin or your bathtub. Use the sponge to wipe the pack's interior and pockets out.
The key to scrubbing is to only focus on the areas that are the dirtiest or accumulate the most sweat, like the straps, hip area, and back panel. Since there are parts that are specially treated on your bag, only scrubbing the problem areas will help preserve them. Pour a bit of your soap—which could be castile soap or Nikwax TechWash—onto your sponge and lightly scrub until clean. Then, switch over to your soft brush to clean out the build-up in your pack's zippers. The soft brush will also work well on embroidered areas, where grime can build up between the threads.
Once the backpack is fully cleaned, run cold water over it to rinse off the soap. If it was particularly dirty, it may need to be rinsed twice. Move it aside to allow it to air dry as you clean your detached parts, if applicable.
If your pack is older, it may need to be sprayed with a water repellant. That way, it can still carry on as your trusty backpack. When you're finished, open all pockets and zippers to prevent mustiness from damp areas.
It's important to note to never machine dry your pack or under any type of high heat. Direct sunlight could also discolor your tactical backpack, so a low-lit area will be best. Packs only need this type of cleaning once or twice a year. Otherwise, a quick wipe down with a damp cloth will suffice. Once it's completely dry, you can re-assemble it and get ready for your next outdoor adventure!