you need to click the little 'radio button' for 'spend miles' just under the place you enter route and dates and travel class.

If you wanted to upgrade from PE to UC then choose UC as that will tell you if there are any reward seats available. If there are no UC rewards then you can't upgrade. Upgrades from Y and PE into UC come from the same inventory of seats

But to upgrade from PE you also need to be in either the W or S fare buckets

You can do this search under the 'spend miles' tab then the 'upgrade/companion flights' tab in the flying club section of the website.

But you might as well phone VS as you would need to phone them anyway to process the miles upgrade and they will be able to check if there are any seats in UC that you can upgrade into