Up On = location that is elevated (e.g. Up on the mountain, there is plenty of fresh air.)
Upon = a formal and antiquated version of the word ‘on’, and sometimes “used to show that something happens soon after, and often because of, something else.”* (e.g. Upon seeing her test grade, she contacted the teacher to see if there had been a mistake.)
When used between two nouns that are the same, ‘upon’ indicates the large number or amount of the thing that has been mentioned.** (e.g. She saw miles upon miles of yellow canola fields as she looked out of the car window.)
*Cambridge dictionary, http://www.dictionary.cambridge.org/dictionary/english/upon, Accessed September 2, 2019.
**One Stop English, http://www.onestopenglish.com, Accessed September 2, 2019.