"Backing up" normally means making a copy of data in separate media so that, in the event of a problem with the original data, the files can be restored from the backup. One of the best programs for doing this is Acronis True Image.
If you just want to transfer files from the "C" drive to an alternative, such as a USB external drive, to free up space, the files can simply be copied over and then the originals deleted from your "C" drive.
There are also synchronisation programs, such as "AllwaySync", but this isn't going to free up space on your "C" drive if it is filling up.
_________________
Mike