Every file type in Windows 8 is usually associated by a default program. When you double click a file type the default program is chosen to open that particular file type. However, you can change this default program by using any of the below given methods:
1) Method-1: From file:
i) Right click on the file for which you wish to change the default program.
ii) Go to: Open with and click on “Choose default program”
iii) Check option: Use this app for all TYPE files. (where, TYPE is the type of the file in question).
iv) Click on “More Options” and select a default program.
2) Method-2: From Control panel:
i) Go to: Charms bar –> Settings –> Control Panel
ii) Select “Small icons” for “View by” drop down menu on the right side.
iii) Click on “Default Programs”
iv) Click on “Associate a file type or protocol with a program”.
v) Select the nae of a file on the left side, then click on “Change Program”.
vi) Click on “More Options” and select a default program.
vii) When, you are done, click on “Close”.