You need to create your username and password to login to the GST Portal. For subsequent logins, you will use the username created now.
Your password should be of 8 to 15 characters which should comprise of at least one number, special character and letter (at least one upper and lower case).
Post grant of GSTIN, when you login for the first time on the GST Portal, you will be prompted to file a non-core amendment application to enter Bank Accounts details.
You can file a non-core amendment application to enter Bank Accounts details.