Instructions
Open the Start menu on your computer, and select "Control Panel." Choose "Folder Options."
Click the "View" tab, and uncheck the box next to "Hide extensions for known file types." Click "OK."
Navigate to the SMC files you need to change to NES. Highlight the file by clicking on it.
Click on the end of the highlighted filename to place the cursor after the name. Delete the letters "SMC" and replace them with "NES."
Press "Enter" or click elsewhere within the folder to confirm the change.