TAKE A BACK UP FIRST OF YOUR EXISTING FILES IN YOUR BACK UP IN CASE SOMETHING GOES WRONG

Then from a command prompt do:

xcopy [path to directory] [path to where you are saving it to] /D /Y

That will copy any file in your My Docs folder that is newer to your back up medium and overwrite the older files with newer ones and add the newer files to that directory (the /D bit). It will overwrite without prompting so be careful (the /Y bit)