I want to override my CiviCRM settings. I am new with CiviCRM and I don't know where to put the line referenced in the attached image. Thanks in advance.
I believe you need to update the directory "/var/www/testing/sites/default/files/civicrm/upload" to reflect your installation directory:
There may be another subfolder in your setup, like htdocs before the 'sites' folder as well.
You need to add this in civicrm.settings.php. Once you have added overided the settings clear CiviCRM cache so that the settings are updated correctly.
I think this could be an issue with your server settings w.r.t. the permissions on that file. basically it is saying 'you don't have permission to edit/save this file' so you either needs to fix them or use sudo