how could i connect to lambda to rds

If you mean by lambda AWS Lambda and by rds Remote Desktops Services, then the answer you cannot.

AWS Lambda is an event-driven, serverless computing platform. RDS is a user interface.

So, please rephrase your question, provide some context.

Hi,
check this official document by aws.
you need to create a  aws role that can access RDS instance via vpc through lamda.
https://docs.aws.amazon.com/lambda/latest/dg/services-rds-tutorial.html

Create an IAM Role which has access to RDS
Use that role in Lambda function to access RDS

There are actually 2 steps to do.

1. Allow the Lambda to access RDS by attaching a proper policy to the lambda execution role. 
2. Bundle your Lambda with the modules that support RDS access. 

Consider the following Lambda python code example:

import pymysql
import sys

db_username = "someuser"
db_password = "somepassword"
db_name = "mydatabase"
db_host="mydatabase.mydomain.co"
db_port = 3306

def main(event=None, context=None):
    try:
        conn = pymysql.connect(
            db_host, 
            user=db_name, 
            passwd=db_password, 
            db=db_name, 
            connect_timeout=25, 
            autocommit=True
         )
    except:
        print("ERROR: Unexpected error: Could not connect to MySql instance.")
        sys.exit()

    print("SUCCESS: Connection to RDS mysql instance succeeded")

if __name__ == '__main__':
    main()

Select allOpen in new window

The pymysql module is NOT part of the python runtime, so you have to either bundle it with your lambda function, or place it as a layer.

Bundling is easy. Start a shell/command line in the directory where your code is, and execute a command that will place all of the requirements in this directory. In python it will be

pip install pymysql -t .

Select allOpen in new window

then just zip your lambda with all the subdirectories and upload as a zip file.

Creating a layer is a bit more complex, read about it here.
https://docs.aws.amazon.com/lambda/latest/dg/configuration-layers.html