Open you 2017 Turbotax file, select "Federal Taxes" then select "Wages and Income" tab, and then select "I'll Choose What I Work On". When the "2017 Income Summary" page opens, scan down to "Business Items" and select the "Update" tab for "Business Income and Expenses". When the "Business Summary" page open, select 'Delete" for the old Schedule C and it will be removed from the 2017 return.
Note: If you depreciated assets in a prior year Schedule C and the depreciation is carried forward, Turbotax may ask what you want to do with any remaining depreciation. You may want to get further help if you have remaining depreciation.