You would need to download the 2018 tax data file from the online return and then open the data file with the desktop TurboTax software to continue the 2018 tax return.
See this TurboTax support FAQ for saving a tax data file using the online editions - https://ttlc.intuit.com/questions/1899645-how-do-i-save-my-2018-turbotax-online-return-as-a-tax-data...
