In Windows 7, you can specify a program for a file type using the Default Programs section available as part of the Control Panel.
Click Start –> Control Panel –> Programs.
Under Default Programs section, click the Make a file type always open in a specific program link. This should display the following Set Association window.
Now if you want to change the program associated with a text file to a WordPad (.txt extension) then scroll down and select the .txt file extension and click on the Change program button. This would display the following Open with window. Now select WordPad and click Ok button to confirm the changes.
If you are looking for any other program to be associated with a file type and if it is not listed under the Recommended Programs or Other Programs section then you can use Browse button available at the bottom of the screen to locate the Program.