If you're referring to Adobe Flash Player Standalone, remove the software using Add or Remove programs feature (Control Panel > Uninstall a program), or try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system. After that, download and install its latest version.
If you're referring to Adobe Flash Player plugin, use the uninstaller provided by the support team and follow the instructions from the official web page. After that, visit the Adobe Downloads page, click the yellow Install now button, save the file to your PC, close your web browser and install the tool following the on-screen instructions.