Startup costs for a C-Corp are entered in TurboTax Business under Federal Taxes >> Deductions >> Business Expenses. Return to this topic and update the information to delete your entry for start-up expenses.
If you capitalized any portion of these costs, go to Depreciation of Assets and remove the entry for start-up costs.
However, if you reported these start-up costs in a prior year, you cannot remove them on the current year return. You would need to amend the return(s) where the costs were expensed or amortized.