If you are unable to uninstall an already installed extension then there is likely some sort of corruption of your configuration folder. Unfortunately the way to get it in a working order will also uninstall all of your other extensions in the process. You will need to clear out your configuration folder then afterwards remove the extensions that you see left over in the extension manager. Here is a link to a post that details exactly where and how to do this, please give this a try. Afterwards you should have no problems installing this new version or any other extensions.