"Onto" and "upon" can be synonymous: He leaped upon the chair. or He leaped onto the chair. (the latter seems more common). However, in uses such as: "Upon learning that his wife was unfaithful, he cried." "onto" would not be used. "Onto" exclusively expresses a physical location. "Upon" can be used colloquially to express a moment in time.

The only time I see 'upon' regularly is in the traditional beginning of a fairy tale: Once upon a time...

We don't use "upon" much. Use "onto" most of the time. (Or use "up on" (two words) in many cases.) He jumped up on the couch. (We would not say "He jumped upon the couch." Sounds weird.)