I see two issues:
1.
You have a typo (missing '=') in
Session_Year:Session_Year
2.
you shouldn't have a variable directly in your SQL statement ($pNo at the end).
You probably did try with one more parameter, but if you were using the same parameter name, that would fail as all parameter names (not the variables they are bound to) must be different. So you could choose a different parameter name.
But then, there is no need to "update" pNo anyway, as it already holds the right value due to your WHERE clause, so leave it out of the key-value pair list, and that problem no longer exists.
:)