'Modifies the specified Reserved Instances.
:type client_token: string
:param client_token: A unique, case-sensitive, token you provide to
ensure idempotency of your modification request.
:type reserved_instance_ids: List of strings
:param reserved_instance_ids: The IDs of the Reserved Instances to
modify.
:type target_configurations: List of :class:`boto.ec2.reservedinstance.ReservedInstancesConfiguration`
:param target_configurations: The configuration settings for the
modified Reserved Instances.
:rtype: string
:return: The unique ID for the submitted modification request.'

def modify_reserved_instances(self, client_token, reserved_instance_ids, target_configurations):

    params = {}
    if (client_token is not None):
        params['ClientToken'] = client_token
    if (reserved_instance_ids is not None):
        self.build_list_params(params, reserved_instance_ids, 'ReservedInstancesId')
    if (target_configurations is not None):
        self.build_configurations_param_list(params, target_configurations)
    mrir = self.get_object('ModifyReservedInstances', params, ModifyReservedInstancesResult, verb='POST')
    return mrir.modification_id
