I would like to use Geany as my local editor in GitHub Desktop instead of the default Notepad++. I dont see an easy way to change this default.
At this time GitHub Desktop only supports editors that have been specifically integrated into the application. You can find the details of how this works along with the supported editors here:
We happily accept pull requests to integrate new editors. There is also a tracking issue where we are collecting feedback about better support for external editors:
Please feel free to add feedback there. I hope that helps!