Change default app

[ElAguila]

How do you change a default application for opening a file type. I am evaluating several programs for viewing my pictures. I had no default defined so that when I tapped a jpg file I got a popup menu asking which app I wanted to open it with. Someway or another I managed to check the box that says use this as the default and tapped the app before I could uncheck it. So now the built in picture viewer is set as the default and I would like to clear that so I get the popup box again. Is there any way to do this?