Why do I have a box on my head/hand

From Second Life Wiki

This happens to all of us! Some items that have others inside (like freebie boxes, or packages containing clothes, vehicles, etc.) should be dropped on the ground instead of "attached" to you.

Right-click on the attached box and select "Drop". That will do the trick. Next time, just open Inventory, locate the box you wish to open, and drag & drop it on the ground. That's all!

To look at the items inside the box, right-click on it (when it's on the ground) and select "Open".