'Updates an Amazon RDS instance.
**Required Permissions**: To use this action, an IAM user must
have a Manage permissions level for the stack, or an attached
policy that explicitly grants permissions. For more
information on user permissions, see `Managing User
Permissions`_.
:type rds_db_instance_arn: string
:param rds_db_instance_arn: The Amazon RDS instance\'s ARN.
:type db_user: string
:param db_user: The master user name.
:type db_password: string
:param db_password: The database password.'

def update_rds_db_instance(self, rds_db_instance_arn, db_user=None, db_password=None):

    params = {'RdsDbInstanceArn': rds_db_instance_arn}
    if (db_user is not None):
        params['DbUser'] = db_user
    if (db_password is not None):
        params['DbPassword'] = db_password
    return self.make_request(action='UpdateRdsDbInstance', body=json.dumps(params))
