You already own Opus 12 (Light) and don't need to pay anything. It sounds like you just have not installed it yet, and are still using Opus 10.
You just need to download and install Opus 12 to start using it.
From Common purchasing, upgrading and licensing questions :
I upgraded to Opus 12 but it still says I'm using Opus 11!
You probably just need to install Opus 12
Upgrading your licence isn't enough, you need to install the new program as well. You can download it here.