Relief under sec 89 for arrears

You will have to file a revised return mentioning the relief in the ITR, subject to revised ITR is filed within its due date.

To save you from any additional tax burden due to delay in receiving income, the tax laws allow a relief under section 89(1). In simple words, you do not pay more taxes if there was a delay in payment to you and you were in a lower tax bracket for the year you received the money. For claiming relief under section 89(1) for arrears of salary received, it is mandatory to file Form 10E with the Income-tax department. If Form 10E is not filed and relief is claimed, then the taxpayer is most likely to receive notice from the Income-tax department for not filing Form 10E.

Revised ITR can not be filed now, You can file for rectification 

If in case of receipt of past salary, salary in advance or receipt of family pension in arrears, you are allowed some tax relief under section 89(1). Step 1: Calculate tax payable on the total income, including additional salary – in the year it is received. Arrears provided will reflect in Part B of Form 16.