In case you have a Dineout Passport membership activated through Times Prime, the contact number in the account cannot be updated.

If that's not the case, ensure that the number/email that you are trying to update is unique, and not being used in any other Dineout Account.

In case it is already registered in another account, follow these steps: 

Logout of the current account > Login into the account in which the required number/email is registered > Go to My Account > update a third email/number so that the required email/number can be freed > Once updated, you can login to the original account & update the required number/email