Each file extension in Windows is associated to a program and when you open the file, its set to open with the default program for that particular extension. You can set/change the association of each extension with any program manually. To do this Open Control Panel and select “Make a file type always open in a specific program” option under Default Programs.
Here you can see the list of all extensions and the associated programs on the right.
If you need to change or set the default program, select the extension and click Change Program button located on the top right corner.
Here the recommended programs and also the other programs are displayed. Once you select a program, the file will be associated with that program. Alternatively if you need to change only for a particular file, right click on the file and select Properties-> General Tab and select the Change button.
Once you are selected the program, click OK.