B&R 9.5 U3a release notes tell me to download the 9.5 ISO image and do the upgrade. It's a bit unclear to me if this means I need to upgrade to an older version of 9.5 first or if the full ISO for 9.5 U3a can do the upgrade from 8.0 U3 directly. I'm guessing the latter but I'd rather not guess
Downloading the latest full ISO and running the installer will first install 9.5, then update to U3a in the same run. Just went through a legacy upgrade myself not too long ago.