Ideally you should wait until your original return is fully processed before amending to include the additional interest income. But, at minimum, you should wait until your return has been accepted by the IRS before amending. If for some reason it is rejected, then you can just add the interest income before re-submitting your return.
Take a look at the following TurboTax help article for more information about amending your return:
How do I amend my federal tax return for this year?