At the bottom right corner of the game window is an icon for the backpack. Click on this once to open it.

Once your backpack is open, you will notice a small icon of the backpack on the top left side of the window. Next to this backpack icon is a small round icon. By clicking this icon you can change the style of backpack you see on the screen, as seen below.

If you drag an item such as bandages from your backpack, it will not give you an option for quantity. To do this, hold the shift key down as you drag the item from your backpack.