I have to prove that if all partial order has all least upper bounds, then it alos has all greatest lower bounds and conversely. (which im guessing is talking about complete lattices)

I have no idea how to go about this proof. I guess my question is : how can I represent that to myself? From what I understood, lubs and glbs are unique when they exist. So how can a partial ordering have "all" lubs or "all" glbs?

If someone could help me with this first question, that would probably help me a lot to write the proof