I would like to change my rescue email id though I forgot security questions

You won't be able to change it until you get the answers to your Security Questions reset. Contact Apple Support to get them reset:

ACCOUNT SECURITY CONTACT NUMBERS

Cheers,

GB

You can't change it unless you can access the Password & Security section of applied.apple.com. And you can only do that by answering your security questions, or with Apple's assistance:

How to reset your Apple ID security questions.

Go to appleid.apple.com, click on the blue button that says 'Manage Your Apple ID'.

Log in with your Apple ID and password. (If you have forgotten your Apple ID password, go to iforgot.apple.com first to reset your password with a password recovery email)

Go to the Password & Security section on the left side, and click on the link underneath the security questions that says 'Forgot your answers? Send reset security info email to [email]'. This will generate an automated e-mail that will allow you to reset your security questions.

If that doesn't work, or if there is no rescue email link available, then click on 'Temporary Support PIN' that is in the bottom left side, and generate a 4-digit PIN for the Apple Account Security Advisor you will be contacting later.

Next, go to https://getsupport.apple.com

(If you see a message that says 'There are no products registered to this Apple ID, simply click on 'See all products and services')

Choose 'More Products & Services', then 'Apple ID'.

A new page will open.

Choose 'Other Apple ID Topics', then 'Forgotten Apple ID Security Questions'.

Click the blue 'Continue' button.

Select the contact option that suits your needs best.