First of all, I saw that the Dual Sternum Strap Adjuster of the backpack is a small object with the same Japanese character, with two grids up and down. It can be used to adjust the length of the backpack. When wearing, first lay the strap of the backpack flat through the lower compartment of the Dual Sternum Strap Adjuster, then stand up the side of the sun button and then pick up the strap of the backpack and pass it through the upper compartment of the Dual Sternum Strap Adjuster. After wearing, in order to make the strap of the backpack more stable, pass the strap of the backpack through the lower grid of the Japanese buckle.

Next, you'll need to fine-tune the straps for a better fit. Loosen the straps first, then gently pull the ends of the straps forward, then back and down for a more natural fit. Be careful not to stretch too hard and not put too much weight on your shoulders.
If adjusted correctly, the straps should fit perfectly around your shoulders, with no gaps between the straps and your back. If there is a gap, it is likely that your piggyback system is set too long. Shorten the piggyback system a bit and try again.