It means the first one, or in a vertical position, vertically since it's used as an adverb in this example. That's the literal meaning. Basically, it's implying not to store it sideways. Also, just using common sense (aside from the dictionary meaning), it suggests to me not to store it upside down (if there is such a distinction).
Not like this:
The just in that definition refers to (M-W):
2 a (1) : acting or being in conformity with what is morally upright or good : righteous • a just war
It is synonymous with "honest, honourable" in the definition you cited. It is not meant as
2 a : by a very small margin : barely • just too late
b : immediately, directly • just west of here