GST User ID once we created we can't change that in future. But Password we can change if we have register mobile no or email id. If you don't have both then Contact your local sales tax office to change your password.
GST User ID can not be changed but password can be change through email id and register mobile number, however if you do not have both you can go to Jurisdiction officer with request letter to change password. after verification your original documents Jurisdiction officer will change password. once you got your new password, you can login to GST portal and after that you can change new email id and mobile number on portal.