a doubt

[Ox_mx]

How could I use "upon"?

[Tdol]

How could I use "upon"?

It isn't used that much. We can use it instead of 'on' sometimes:

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