If you go to Deductions & Credits, click I'll choose what I work on, and then scroll down to click Done with Deductions. 

You will have to click through a few screens, such as the summary screen for your deductions and credits. 

The Standard/Itemized deduction screen should come up soon after, along with a small box to check if you'd like to change your deduction from standard to itemized. 

You can see how both deductions affect your return, and you can always switch back at any time should you change your mind.

Now two years later, but I tried your suggestion. Unfortunately, TT's calculations do not revert back when I try to 'undo' my initial change. Ideas?

Click this link for info on How to Switch from Standard to Itemized Deductions (or vice versa).

If your calculations don't change, if you're using TurboTax Online, close the program, clear your Cache and Cookies and re-open the program.

This link gives steps on How to Preview your TurboTax Online Form 1040. 

Thanks. I spoke with a TT support person who presented my situation to one of their tax experts who confirmed that there is a software glitch that prevents the Federal Tax Due amount from changing (apparently more than once) if you toggle between choosing the itemized vs. standard deductions. I was added to a long list of others who had called on this issue which is apparently being evaluated by 'someone' at TT and there is no ETA on the fix. Stay tuned....

Thanks. Using the desktop version and so already tried closing out and reopening TT. Didn't make any difference. Apparently, there is a link on your Salesforce regarding this issue. Check it out. It has been reported by others. Not fixed yet.