How do I amend a tax return that has already been filed from standard to itemized?

If you need to select Itemized deductions because you are married filing separately and your spouse is itemizing, please follow these steps:

1. Click on the Personal Info tab.
2. In the Personal info summary, click on Edit next to your marital status.  [See Screenshot #1, below.]
3. On the next screen, Let's confirm your marital status, mark the buttons for Married and No you don't want to file with your spouse.  Click Continue.  [Screenshot #2]
4. On the screen, Let's see if these apply, mark the first box, Either [spouse] or I claim itemized deductions.  Click Continue.  [Screenshot #3]

This will force TurboTax to itemize deductions, even if it is not as beneficial as the standard deduction.

If you haven't filed your return yet, or if you've filed, but it's been rejected, just go back into your and make the changes.

If your return has been accepted you can amend your return to itemize deductions -- please follow these steps:

1. Sign back in to TurboTax and go to your Tax Timeline.  [See Screenshot #1, below.]
2. In the section Some Things You Can Do (below the grey line), click on Amend (change) return. [Screenshot #2]
3. On the screen, Are you sure you'd like to amend, click the box Amend using TurboTax Online.
4. On the screen, Have you already filed this return, click the Yes button and then click the button next to I need to amend my 2016 return and click Continue.  [Screenshot #3 ]
5. On the screen, What do you need to change on your return, scroll down and click on the Update box next Deductions & Credits. [Screenshot #4]
6. Continue through the screens, entering the requested information.

This does not help, because it still does not give me the option to choose itemized deductions at any point, it just states that I have chosen standardized deductions even after I go back in and update all of the information within the amended tax deduction area.

It might be that the itemized deductions you entered are less than the standard deduction.  TurboTax will select the method that gives you the greatest benefit--standard or itemized deduction.
If you want to select itemized deductions because you are married and filing separately from your spouse, please see the additional answer posted below.