You can follow the instructions in this TurboTax Help article to switch to TurboTax Online from your TurboTax Desktop.
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You cannot move a started 2024 return from the desktop product to Online TurboTax without starting over, if that's what you are wanting to do. You can only transfer last year's 2023 data file from the desktop into 2024 Online TurboTax.
To switch from 2024 desktop Premier to Online, you'd have to start over in Online TurboTax and would be charged again unless you ask for a product refund for your desktop Premier. The license code for Premier cannot be used to pay for Online TurboTax.
Why would you want to make that move? The desktop Premier is superior and has more features, including Forms Mode to view/edit forms during preparation. Desktop Premier also includes a state program download to prepare a state return. In Online TurboTax there is an additional fee to prepare a state return.
Desktop Premier can prepare multiple Federal returns. Online TurboTax can only prepare 1 Federal return per account per year.