Here's how to download your already-started *.tax2017 data file from your online account, so you can open it and continue working on it in desktop software.
https://ttlc.intuit.com/questions/1899645-how-do-i-save-my-2017-turbotax-online-return-as-a-tax-data...
And also see this:
FAQ:
How do I switch from TurboTax Online to the TurboTax software?
https://ttlc.intuit.com/replies/3302260
