void updateReferencingSheet(Document doc)
{
    // Retrieve the built-in VIEW_DISCIPLINE parameter
    Parameter discipline = selectedViewport
        .get_Parameter(BuiltInParameter.VIEW_DISCIPLINE);

    // Save the value of that parameter for later
    int disciplineNo = discipline.AsInteger();

    Transaction transaction = new Transaction(doc);
    transaction.Start("Updating the model");

    // If the parameter value is set to 1 change it to 2
    // if it's set to anything else change it to 1
    discipline.Set(1 == disciplineNo ? 2 : 1);

    // Now change it back to the previously saved value
    discipline.Set(disciplineNo);
    transaction.Commit();

    // Voila! Your "Referencing Sheet" parameter is up to date
}