Taking a look at the extension you installed - admin/controller/extension/module/xxxx.php and look for
public function uninstall
just check this does not say anything about the user_group ..
1.Copy all files/folder to exact loction manually. (Goto step 6)
2.OR with the help of "Extension Installer" below are the steps.
3.Open your admin goto Extensions Tab and click "Extension Installer".
4.Then click on "Upload" button and select the downloaded folder/directory "testimonials.ocmod.zip".
5.After that goto "Modification" you can see the file below, now click clear and refresh button.