Sometimes the preposition ‘upon‘ can be used to indicate ‘movement from a lower level to a higher level, keeping in touch with the surface‘ :
‘We went upon the hill.‘
But in this case, we can use the preposition ‘up‘ also :
‘We went up the hill.‘
What is the difference between ‘up‘ and ‘upon‘ in the above sentences?
I don't find We went upon the hill particularly idiomatic. Upon usually describes a position rather than movement. It's far more natural to say
We went up the hill
but you can say
We stood upon the summit of the hill, admiring the view.
I wonder where you found that definition. It's misleading, I think.
As Lexico says,
The preposition upon has the same core meaning as the preposition on. However, in modern English upon tends to be restricted to more formal contexts or to established phrases and idioms, as in "once upon a time" and "row upon row of seats"
There are a great many established phrases and idioms, but we wouldn't normally 'go upon a hill'.
You might find the Macmillan (British or American) dictionary helpful.
