Update customer billing address within custom pane
You can update the field value like this:
  $cart_provider = \Drupal::service('commerce_cart.cart_provider');
  $cart = $cart_provider-&gt;getCart('default');
  $billing_profile = $cart-&gt;getBillingProfile();
  $billing_profile-&gt;field_name-&gt;value = 'Hello World';
  $billing_profile-&gt;save();