Drupal is not making it very easy to update themes. This is why we created an external tool which should guide you through a theme update. https://tool-drupal-update-helper.michaelstauner.com/.
These kind of updates should be only performed by technical personell or experts. Theme updates are a nice-to-have but not a must-have. If you want to keep your page secure do a Drupal update. Theme updates are just styling updates.