This is really the wrong forum, but what you can do is find a file of the format of which you wish to change. Left click on the file once. Then HOLD shift and right click it with your mouse. You should then be presented with an option "Open With". Choose that and scroll down to find the app that you wish to re-associate with. Click it. Then in the bottom left corner, make sure "Always use this program to open this type of file" is checked. There you go.