I've moved your question to the InDesign EPUB forum.
The answer depends on your version of InDesign. In InDesign CC 2014 (the current version), it's in the Viewing Apps tab, and you can choose another one by choosing Add Application... Click Remove to remove an application.
The default application mentioned in the InDesign ePub export is the application that opens the ePub on doubleclick, outside of InDesign. Right-Click on an ePub file and change the default open app in the File information/Properties and apply this change to all ePub files. When you restart InDesign, the default app should have changed.