How to Change a Default App to Open Files
To change the default app used to open files, go to Settings/Apps. Then, select the app you don't want as default first. Once inside of that app information, click on Set as default, and clear that app from being default (by pressing the button 'Clear' next to the Clear defaults announcement). Finally, go ahead and open the file you want to read and the message for selecting a new default app will pop up.