I have searched the meaning of look down upon as well as look down on is 'to hate' or to consider someone or something lesser or inferior in some way.
But when we use look down upon and look down on? Thanks in advance.
Your search should have included one or two good dictionaries. The only difference between those two phrases is that "upon" is slightly more formal, and is more likely to be found in British English, than "on".
"Hate" is an inaccurate description of the meaning of "look down on/upon". The phrase, when used figuratively, simply means "to think that you are better than somebody/something".