I am assuming the following.
1. You have installed the program physically on your computer.
2. You used that program to prepare and file your original return with.
3. Your e-filed return has been accepted by the IRS *AND* completely processed. Your return is not processed until you have received your refund, or if you owe taxes the payment has cleared your bank.
If all assumptions are correct, then open the turbotax program. Then click the AMEND button *before* you open your tax return file, and follow the prompts from there.