Short GST charged.

A Debit Note can be issued in a situation where an original tax invoice has already been issued by the seller and the tax (i.e. GST) charged in the original invoice is less than actual tax payable.

Your situation is the same as mentioned above as you have levied GST @ 18% instead of 28%. Accordingly, you shall be required to issue a debit note and report the same under Table 9B of GSTR-1.

You may follow the steps mentioned below:

Go to GSTR-1>Select Table 9B>Click on "Uploaded by Tax Payer Tab">Add Details>Fill mandatory Fields> Click on Save.

Thank you for quick response. When I click on Table 9B there is not option showing Reason for issuing note.

As i see that the contents of the table have been changed, accordingly you may proceed without mentioning the reason.

Okay. Now my concern is that Debit note in GSTR requires me to input Taxable value. But there is no change in taxable value only GST is to be increased

Yes Dear Shubham Bansal,.. you are correct as its only due GST rate difference 18-28 which comes to 10 difference of rate & there is no tax rate of 10% in GST , so it cannot be resolved by issuing Debit note (As your base/taxable amount is correct)

If you have already filed the return :
It's Better to ammend that bill in your next GSTR 1, (send the copy of bill to your client)