You have to file an amended tax return for the year you were self-employed, report your SE income/expenses and therefore eligible for the Qualified Business Income Deduction using Form 8995.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
