In most cases, a clean install is not required when installing Flash Player. The Flash Player installer will upgrade previous installs without having to uninstall first. However, if you find you're running into problems or would like to downgrade to a previous version, the following steps and video should help.
If you are viewing this page in Firefox and Flash Player is not working, please click here for an HTML5 version of this video: YouTube HTML5 HD Link
At this point, Flash Player should be fully removed from your system. To download the most recent version of the Player for installation, please visit http://get.adobe.com/flashplayer