How could I use "upon"?
It isn't used that much. We can use it instead of 'on' sometimes:Originally Posted by Ox_mx
It's on the table.
I put it on the table.
Here, you could substitute it in the second example, though I doubt that I would.
However, it is fairly commonly used in some phrasal verbs and other expressions as an alternative to 'on' or a varient:
Come upon = come across
Upon arrival = on arrival