If you want to install the full version or a newer version, you do not need to uninstall the trial version or older version. The trial or old version is overwritten when installing the full or newer version To nevertheless uninstall ElementsXXL go to Start > Control Panel > Uninstall a program (Windows XP, Vista & 7) or right click the Start button, select Control Panel and click on "Programs and Features" (Windows 8 & 10). On the appearing dialog double click the appropriate item to uninstall it.
Double click the downloaded .exe file and follow the installation instructions. When Plugin Installer shows up, activate the check boxes of your preferred application(s) and press the Install button.
You can run Plugin Installer at any time from Start > All Programs > The Plugin Site > ElementsXXL > Plugin Installer. Then you have more options than during installation, e.g. an Uninstall button.
If you install a new version of Photoshop Elements and ElementsXXL is already installed, you do not need to run the full ElementsXXL installation again. You can run Plugin Installer from the Start menu and use it to install ElementsXXL into the newly installed version of Photoshop Elements.