My current version on Win 10
Directory Opus Pro 10.5.0.0 (4779) x64
OS 6.2 (B:9200 P:2 T:1) SP 0.0
The update (manually or automatically) simply stalls. Never starts, never ends. How do I get it to start and go through
It could be an issue where the old InstallShield installers stopped working on some Windows 10 systems around early 2020.
We aren't sure why, but some people started seeing the issue then, and it looked like the OS (or maybe antivirus or something similar) was breaking the installer at quite a low level. We were never able to reproduce it on any of our systems, so could only do limited analysis via logs people made, but more and more people started reporting the issue as time went on.
In the end, we solved the problem by ditching InstallShield and creating a new installer, which is used from version 12.20 on.
Do I
- uninstall 10.5 first, then install 12
- simply install 12
Much obliged 
That's entirely up to you. If you want the new default toolbars (which have changed a lot in the decade since 10) then uninstall the old version first, so you get the new defaults.