You can make changes to your 2018 tax return in TurboTax as long as you have not filed(e-filed or mailed) your taxes.
If you don't see the toolbar on the left, scroll to the bottom of the Home page, click Add a State(you're not actually adding a state, this just gets you into the right menu), and then over to the left of the screen you should see the toolbar. From there you should be able to navigate between different sections to make changes. To makes changes to your income or to add a W2, 1099misc, etc..., just visit the Federal section. To make changes to your birthdate, name, SSN, address, etc..., just visit the My info section. To make changes to your State tax return, just visit the State section. To pay your TurboTax fees, visit the File tab on Step 1. To make changes to your refund method, visit the File tab on Step 2. When you are ready to file your taxes, select the File tab and navigate to Step 3, from there you can choose between the e-file or file by mail option.