Re: Please tell me how to replace the contents of the bag.

I am on PC but I assume the idea is the same. You pull up the inventory that shows you your personal layout + the content of the backpack. than you click on the content in the backpack that you want to switch to your body. On PC it than opens 2 alternatives: Drop and Swap. You click on Swap. Than  you click on the bodyslot/equipment you want to the equipment with. And it should change the equipment around. At time I need several try to get it to work.

You can only swap within the same category of items so you cannot swap a grenade for a pistol or main gun,

Hope this helps.