You start with the current existing return. Start the Amended process then enter the missing interest.
How to Amend the current year
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...
It's under Federal Taxes Tab or Personal (Home & Business)
Other Tax Situations
Other Tax Forms
Amend a Return - Click Start or Update
