Settings > Preferences, then:
-
For Explorer Replacement:
Launching Opus > Explorer Replacement. -
For the Viewer:
File Operations > Double-click on Files: Use internal picture viewer...
( More detail at How to use the Opus image viewer inside or outside of Opus )
I think those are the main ones you might want to change after installing.
There's a search field at the bottom of the Preferences dialog which can find more things quickly.