For treks that last several days, it is not uncommon to carry bag packs that are more than 15 kg. It is important to use the right technique so as not to strain your back or rip your backpack strap off in the process.
Elevated support
If there is an elevated support such as ledge/step/bar that is about your waist level, lift your backpack with your hands on both straps onto the support. Put your hands through the straps and lift the backpack off the support.
No external support
In most circumstances, your backpack is on the ground and there is no ledge or bench to elevate the backpack. In this case, you will need to use your knee as the support.