if all you want to do is change the file extension go into Windows Explorer, single click on the filename to select it (but not open it), press F2, move the cursor to the end of the name and do the renaming.

But, simply doing this type of extension change is going to result in mostly unreadable crap, the binary transliteration of the DOC file formatting information. If you want to extract the text content of the DOC FILE into a TXT file then in Word do a File, Save As and select file type TXT.