Locate a file of the type you'd like to change the default application for in Finder. Select the file and press Command+I on the keyboard or right click (hold down Control and click) and select Get Info from the contextual menu that appears.
Expand the Open With section in the Info window.
Click the dropdown to select from a list of suggested applications. If the application you'd like to use is not listed choose Other.
Change the Enable dropdown box to All Applications and select your application of choice.
Check the Always Open With box then click the Add button.
Click the Change All... button to use the selected application to open all files of this format.
Click Continue to confirm the change.
All files of the format you selected will now default to opening with the application you've set.