When I did something similar I used brute force:
- I opened the Preferences folder (button bottom left corner of Edit->Preferences)
- Closed Sigil - !Important
- Opened sigil.ini file in a text editor (in my case Notepad++)
- Found the [open_with] entry and removed it completely
- Saved the sigil.ini file
- Started Sigil and added the programs I wanted into Open withs.
If you're using a recent Sigil version, edit sigil_v6.ini
BR