I want to go to the beginning so that I can add my 2017 info. That means I must delete all inputs I have made to the 2018 form. How do I do that?

You cannot use the current online software to prepare a 2017 return.

You can't enter information from 2017 on your 2018 tax return.  Unless you're referring to something else?

Please read this TurboTax FAQ for additional information about preparing a prior-year tax return: How do I prepare a prior-year tax return?

Please read this TurboTax FAQ for additional information about clearing & starting over in TurboTax Online: How do I clear and start over in TurboTax Online?