Do not send an amended return until the original has been fully processed or you will confuse the IRS.
A 1040X cannot be e-filed. It must be printed and mailed. It will take about 4 months for the IRS to process an amended return. It is a good idea to use a mailing service that will track it, like UPS or certified mail, so you will know it was received.
Wait about 3 weeks from mailing and you can check the status of your amended return:
Thanks for the info. My tax-return has already been processed by IRS.
The question I had was - How do I amend the tax return as I did not get $5K from my 2019 daycare FSA (due to missing the deadline).
Your W2 would have shown the $5K in Box 10. With TurboTax, by default, the $5K you contributed to DepCare FSA is included in taxable income unless you match it with a $5K Child and Dependent Care Credit with details on the organization or person that provided the care. This generates a Form 2441. If this step for Child and Dependent Care Credit is the one you forgot, then you just need to open TT to amend your return and go to Deductions and Credits, and under You and Your Family, start/update the Child and Dependent Care Credit section.
Your question is confusing because you stated a deadline of March 31st. This date sounds like a DepCare FSA reimbursement deadline.