I don't see an "Upgrade" button, I see "Re-contract now." Are you sure you are on the right page?
Your best bet is to contact the customer loyalty team directly, you may even get a better deal. Call 0345 172 0046, or try Live chat.
As ferguson has mentioned I would suggest calling our Customer Loyalty Team during Business hours (Monday to Saturday 10:00am to 18:00pm) on 03451720046 or try our Live chat service