The default application is the one you open a file with on doubleclick. In some cases installed programs automatically turn themselves into the default application for files you were happy with, which you might want to change. To do that, rightclick a file of the chosen format and choose Open With... -> Other Application or Open With Other Application:
On the window that opens, choose an application or command and check the Remember this application for "..." files checkbox to apply your selection for all files of the same format:
Hit Open afterwards.
