function my_update_account( $user_id, $user_meta ) {

    $account_id = get_user_meta( $user_id, 'ac_account_id', true );

    if ( ! empty( $account_id ) ) {

        $params = wp_fusion()->crm->get_params();

        $data = array(
            'accountCustomFieldDatum' => array(
                'accountId'     => $account_id,
                'customFieldId' => 2,
                'fieldValue'    => 'NEW VALUE',
            ),
        );

        $params['body'] = json_encode( $data );

        wp_remote_post( wp_fusion()->crm->api_url . '/api/3/accountCustomFieldData', $params );

    }

}

add_action( 'wpf_user_updated', 'my_update_account', 10, 2 );