https://resource.dopus.com/u/mnemic/link-opus
How do I update the current version? I assumed it would update to 13 automatically but it didn't.
How do I set it manually?
The link-page contains information about what it is, but no way to update it, and I can't find any settings for it in user preferences on the website.
It would be great to have the information on how to update/edit/remove your link directly on the link page.
It updates automatically if you upgrade your licence.
You must have instead bought more than one separate licence, so your account is linked to the old one rather than the new one.
I've unlinked your account, which means you can now link it to the new licence.
I did indeed! Thanks for unlinking.
I've had enough bad experiences with upgrades. If I wasn't happy with Dopus 13 I wanted nothing to mess with my existing setup or licence, so I figured I'd do a clean slate 
Never trust updates no-one asked for...
Understandable! Although Opus 13 licences let you install Opus 12 as well, so there's no need to buy a new one rather than upgrade just to keep access to the old version, unless you want two separate licences.