If you have user I'd and password, you can change the mobile and Email I'd by amending the gst registration.
Or if you don't have user is or password....
The email and mobile number can be updated by the concerned Jurisdictional tax authority of the taxpayer as per the following procedure:
Steps to be followed:-
Taxpayer is required to approach the concerned jurisdictional Tax Officer to get the password for the GSTIN allotted to the business.
. Allotted jurisdiction is displayed in red text>
Taxpayer would be required to provide valid documents to the tax officer as proof of his/her identity and to validate the business details related to his GSTIN.
Tax officer will check if the said person is added as a Stakeholder or Authorized Signatory for that GSTIN in the system.
Tax officer will upload necessary proof on the GST Portal in support to authenticate the activity.
Tax officer will enter the new email address and mobile phone number provided by the Taxpayer.
After upload of document, Tax officer will reset the password for the GSTIN in the system.
Username and Temporary password reset will be communicated to the email address as entered by the Tax Officer.
Taxpayer need to login on GST Portal https://www.gst.gov.in/ using the First time login link.
After first time login with the Username and Temporary password that was emailed to him, system would prompt the taxpayer to change username and password. The said username and password can now be used by the taxpayer.