Preprocess Panels or Panelizer content
There is a template_preprocess_panels_pane

/**
 * Implements preprocess_panels_pane().
 */
function YOURTHEME_preprocess_panels_pane(&amp;$vars) {
}


Documentation

If you want to change the output of a field you have:

/**
 * Implements hook_panels_pane_content_alter().
 */
 function YOUR_MODULE_panels_pane_content_alter($content, $pane, $args, $contexts) {
  $content ...
}