This is from the Finder help files:
You can select a specific application to open a document. You may want to do this if you prefer using a certain application or if you don't have the application that was used to create the document.
Select the document in the Finder and choose File > Get Info.
Click "Open with" in the Info window.
Choose an application from the pop-up menu or choose Other to locate a different application.
To use the application you chose to open all similar documents (the same type as the current document and created by the same application), click Change All.