Use the following code to change a field value after a Woocommerce order is completed. Replace 25 with the ID of the form to update, replace 100 with the field ID to update, and replace 'Complete' with the new value you would like to use. Please note: You must have a default value submitted in the field in order for this function to work.

add_action( 'woocommerce_order_status_completed', 'update_frm_entry_after_wc_order_completed' );
function update_frm_entry_after_wc_order_completed( $order_id ) {
	$order = new WC_Order( $order_id );
	$items = $order->get_items();
	foreach ( $items as $item_id => $product ) {
		if ( isset( $product['formidable_form_data'] ) && is_numeric( $product['formidable_form_data'] ) ) {
			$entry_id = $product['formidable_form_data'];
			$entry = FrmEntry::getOne( $entry_id );

			if ( $entry && $entry->form_id == 25 ) {
				$field_id = 100;//Replace 100 with the field ID to update
				$new_value = 'Complete';
				FrmEntryMeta::update_entry_meta( $entry_id, $field_id, null, $new_value );
			}
		}
	}
}