If you are being prompted to upgrade to TurboTax Deluxe, the system is detecting that your tax situation requires additional forms or schedules that are not covered in the free edition.
For additional information about being prompted to upgrade, please see Why do I have to upgrade from Free Edition?
After reviewing the above and you feel your situation still meets the requirements for TurboTax Free Edition, you can switch back by using clear and start over to erase your current return and start from scratch. You can start over as long as you have not registered the product, submitted payment, or deducted your TurboTax fee from your refund.