As a US citizen, she must use Form 1040 to report her taxes. Form 1040NR is only used by non-resident aliens.
Use her Indian address on the 2020 tax return. This is entered in the "Mailing Address" interview of Personal Information. This is the address the IRS will use for correspondence related to her return.
The taxpayer is responsible for complying with the filing thresholds of Form 8938. You don't have "tell" the IRS you are living abroad, but you are required to comply with the appropriate filing threshold.
There is no reason or requirement to update a previous year return to correct an address. Just file the current year return with the correct address.
