First, check the status of your return (e-file status lookup tool)
If you have already e-filed your return, then you will need to wait until the IRS has either accepted or rejected that return before you can amend it to make any changes.
If they reject it, then just make the needed changes and re-submit the return.
If accepted, and you are expecting a refund, wait until you have received your refund, then you can go here and get instructions for amending that return. Amended returns have to be printed and mailed in and take up to 16 weeks to be processed.
One important note: Before you start the process of amending the return in TurboTax, be sure you have printed a copy of your original return or have saved a PDF copy to your computer.