Install over the previous Free version. The newer files will overwrite the older ones - in the chronological sense.
Incidentally, f you compare the installed files in the ..Program Files\Astonsoft\DeepBurner folder with those extracted from the Free Portable zipped version, you will see the similarity in content. The install routine just makes it simple to place the Program files in location and create shortcuts in the Start Menu. This is done manually in the Portable version. In either case DB Free can be run as a standalone program which means it's slim, simple and, as we know, effective, a KISS ('Keep It Simple Stupid').