Editing a users custom fields from a plugin action Craft 3
You should just reference the field as an object property of the user instead of using setFieldValue. I also use the getElements function instead of using the property, but I don't think that makes much difference. The code below should work.
public function actionUpdateuser() {
    // Update user data
    $userId = 67852;
    $user = Craft::$app-&gt;users-&gt;getUserById($userId);
    $user-&gt;userCompanyName = 'New Name 2'; 
    $result = Craft::$app-&gt;getElements()-&gt;saveElement($user);

    return $result;
}

Also make sure to add the Craft Users class at the top of the page with:
use craft\elements\User;