Hi Petter
Infact, the latest version on server is now 1.6.6, not 1.6.5. It contains some javascript bugs fixes in version 1.6.5 but I haven't make an announcement about that version yet.
Since 1.6.6 contains so many changes compare to version 1.6.3, you should back up your site first before running the upgrade. After back-up, you can install the new version over the old version.
Regards,
Tuan