I am on H1b with NRE account. What tax year shall I consider to calculate the total interest earned ? Will it be Indian tax year (April 2018- March 2019) or US tax year?

It will be the US tax year.  January 1, 2019 - December 31, 2019

Thanks for the prompt reply.
For reporting the global interest earned, I aggregated the interest earned over a period of one year based on Indian financial year since it is earned in India while filing the tax return. The return is already is filed. Shall I amend it or there  is a provision to rectify it while filing the next year's return?

You should amend your return.  However, if it doesn't make much of a difference in your tax, you can probably wait to next year to correct it.

Please see below for instructions on how to file an amended 2019 return. You cannnot amend your return until the tax authorities have processed your return and you have received any refund or paid any balance due.

How do I amend my 2019 TurboTax Online return?