If you are using TurboTax Online, try starting from the Wages and Income section and then scroll to the Rental Properties and Royalties. Click Add/Edit and then click the pencil icon next to the property information you have already entered. The next few screens should take you through all of the characteristics of the property, including whether there were any personal use days for the year. At that point, you can make any changes.
If you are using a CD/downloaded version of TurboTax, you can find the personal use days under the Property Profile section in the Rental Income and Expenses part of your return.