Change file description property

Hi, I cannot change the description of some files I have downloaded. I have even tried the instructions given here. I can find the Properties in the Details tab and the Details Pane, but I cannot change them. I am the only user, and administrator, on this home PC. Why can't I change description properties of my own files?

For reference, the files I've downloaded and want to change are FreeFEM++ (an ODE solver language) and FreeFEM++cs (an client-server IDE for FreeFEM++).

Thanks to any helpers.