changing of contact details

Yes, we can change the contact details on GST portal by filing non-core field amendment (available under Services-Registration) after updating the contact details required by you. The contact details should be amended for the authorised signatory, whose contact details can be changed by changing the same in promoters/partners tab in non-core amendment application. Once the changes are made, the application needs to be submitted by verifying the application in "verification" tab in the application.

Yes, you can update the contact details by amending the gst registration.