Here is the simple steps for Updating of Bank Account:

Login on GST Portal>Services>Registration>Amendment of Registration- Non core fields>Bank Account>Mention the following details:

1. Account Number

2.Type of Account

3. IFSC Code

4. Upload Any of the following: Bank statement, Cancelled cheque or First page of Bank Passbook

5. Date of Amendment

6. Reason of Amendment

And then proceed for file