Renaming and copying the rockbox directory is like a clean install, but you still keep a copy of the old directory in case you want to recover something later (installed themes, custom settings, radio presets...)
If you want to do that, plug the player into the computer and rename the .rockbox directory to something like .rockbox-old. Then download the release you want from the Rockbox site, it should come as a zip file. Open the zip file and drag and drop the .rockbox directory inside to your player. Unplug the player safely and reboot it.