Step 1
Click the “Start” button on your desktop and choose “Control Panel” on the right task pane.
Step 2
Click the “Appearance and Personalization” option in the dialog box, then click “Folder Options” on the right task pane. Click the “View” tab on the Folder Options dialog box.
Step 3
Click the “Hide Extensions for Known File Types” check box to remove the check mark, if the box is checked, so you can see the file extensions. Click “OK.”
Step 4
Click the “Start” button. Type a new filename for the file that you want to change in the “Search for Programs and Files” field and press “Enter.” You can use wildcards in the search field and search for all DAT files by typing “*.DAT” or you can type the name of the file.
Step 5
Right-click on the filename and select “Rename” from the drop-down list. Replace the “DAT” file extension with “DOC” and press “Enter.” At the computer warning prompt, click “Yes.”
Step 6
Launch Word and try to open the file again.

I hope this helps.

Thank You!
Lokesh