The backpack should be adjusted when it is fully packed. Place it on a raised platform (e. g., a nightstand) and put on the shoulder straps alternately. Start with the lower thigh strap. It should be tightened so that it rests on the upper half of your thighs, fits snugly, and takes most of the weight. Next, you need to adjust the shoulder straps. Do not tighten them too much so that they don't take the load from the hip belt, dig into your neck, or interfere with your breathing.
After that, I recommend walking around for a while to understand whether you are comfortable or not. It is especially important if you are new to packing and adjusting backpacks. Adjust the straps again if necessary, and if something inside is pressing on your back, remove the pack and redistribute things.