if you’re an Absolute 2 or 3 license holder and would like to upgrade to Groove Agent 5, I’m afraid I have bad news for you: On their website, Steinberg has announced the release of Absolute 4 for January 2019. That would be 2-3 months from now! Steinberg has also posted a note, not to upgrade to Groove Agent 5 directly, but to wait until January for the release of Absolute 4.
Here is the link to the Steinberg page (for the Absolute 4 release announcement): https://www.steinberg.net/en/products/vst/absolute/start.html. I have also included a screenshot.