Believe it or not, Microsoft makes a pretty easy to use little app/program called SyncToy that can be used to backup files to an external hard drive. It is the progam that I use for this purpose. When backing up your files it looks at the date the file was last modified and if the file on your HD has a newer modified date than the one on your external HD (or wherever your backup destination is) it will overwrite that file with newer version. I'm no Microsoft fanboy, but if you are running Windows, SyncToy is a good, simple app for backing up files.