You can only uninstall a package that has been installed. So open the package (right click - "Open"). You will see the package's definition. If you switch to "Statistics" you will see what will happen if you uninstall the package.

To uninstall the package click "Uninstall package" and confirm the dialog. And the uninstallation (which is the installation of an uninstallpackage) starts. This can take some time (even minutes for big packages), so please wait until the uninstallation has finished. To see that something is happening an overlay dialog appears.

After the uninstallation the stored uninstall package will be deleted so the only possible action is to edit the package definition or download a backup package.