You change your filing status in the My Info section of the online program. You need to edit your personal information and change your status from Single to Married and then indicate that you will not be filing the tax return with your spouse.
As an FYI - See this TurboTax support FAQ for filing jointly versus separately - https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/better-married-couple-file-join...
