Depending on the type of income you listed as the additional source, you will need to the delete the form that corresponds to it. So, if you added an additional W-2 for instance, you will need to delete that particular W-2 form in the delete form section of TurboTax.
Listed below is the general instructions for how to delete a form in TurboTax online. Once you see the additional source of income you listed, click delete next to it. Turbo Tax will walk you through the rest of the delete process.