Only the additional income you are adding. You start with your original return and click on Amend. The refund monitors at the should reset to zero but all your original entries will still be there. Then you only enter the new info you are adding or delete items.
You should not amend until the first return has fully processed and you get the first refund or your tax due payment has cleared. But first be sure to save a copy of your original return since the amended return will overwrite it.
You can find Amend under
Federal Taxes or Personal (for Home & Business)
Other Tax Situations
How to Amend this year
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-current-year/L7eS6o1qh_US_en_US