Simply close the viewer, and delete the settings file which location depends on your OS: see
this Wiki page which gives the logs/ file directory location, the settings file (settings_coolvlviewer_12620.xml for the v1.26.20 branch) being located in the user_settings/ sub-directory, present at the same level as the logs/ sub-directory.
Once you are satisfied with your settings, you may save them in a different place (so to restore them easily by copying the saved file over the current settings file), via the "Advanced" -> "Save current settings to file..." menu entry.