Go to My Info and click Edit by Filing Status. It will ask you to Review your Filing Status, click Continue. Then on the next screen you pick if you or your spouse itemized deductions.
See this thread on changing Standard Deduction or Itemized Deductions.
https://ttlc.intuit.com/community/taxes/discussion/how-do-i-file-married-filing-separately-claiming-the-standard-deduction-when-turbotax-says-my-spouse/00/840584