How do I rewrite a facet item label using facetAPI bonus?
This snippet forces the output to be uppercase by using drupal_strtoupper($item["#markup"]);. And then sets $build[$key]["#markup"] to this new uppercased value.

If you need to check for a specific item, you could use:

function HOOK_facet_items_alter(&amp;$build, &amp;$settings) {
  if ($settings-&gt;facet == "YOUR_FACET_NAME") {
    foreach($build as $key =&gt; $item) {
      if ($item["#markup"] == 'my value') { // Check if this is the right item
        $build[$key]["#markup"] = 'my overridden value';
      }
    }
  }
}