Preposition

1. Being above and in contact with another.

   Place the book upon the table.

2. Being directly supported by another.

   The crew set sail upon the sea.

   She balanced upon one foot.

3. At a prescribed point in time.

   The contract was rendered void upon his death.

Adverb

1. Being the target of an action.

   He was set upon by the agitated dogs

2. Incidental to a specified point in time or order of action; usually combined with here-, there- or where-.

   The clock struck noon, whereupon the students proceeded to lunch.