You just use this upgrade page.
Upgrade Protector – Pantheon – Rise Of The Fallen (pantheonmmo.com)
Your profile says you have Supporter access, but I believe the Knight's Pledge from back then is entitled to Champion access. The best way to resolve this is to send a ticket to Support asking them to fix this. They have records of all the pledges and the specific rewards that go with them.
The game is currently in - but very near to the end of - the Seasons testing schedule. This link gives info and access dates. Under Seasons, Champions will next have access to play starting Oct 26. So don't delay getting in touch with Support.
In December, the Early Access program will begin and you'll have more access. This link will explain those changes.
See ya on Terminus