Note that there have been instances of Anti-Virus programs blocking Windows' File Associations. Disabling or uninstalling the Anti-Virus program may resolve problems setting File Associations.
1. Open the Windows Control Panel by clicking on the Windows button and typing in: Control Panel.
2. Select View By: Small or Large icon
3. Click Default Programs
4. Type in the File Extension you want to use.
5. Select the Default App you want to use.