We cannot tell whether you are using online software or desktop download software. If you are using online, you do not have forms mode and cannot override anything. All of your entries must be made via the interview screens. If you are using desktop software, you can make entries using forms mode but you should understand that overriding the entries made via the interview screens will void the accuracy guarantee.
Why are you trying to override a calculation on a TurboTax form or schedule? Why cannot you make changes in the interview section of the program?
Are you sure you really mean override? "Override" means changing a value that TurboTax calculated. It does not mean changing an amount that you entered. Why do you want to change something that TurboTax calculated? What exactly do you want to override?
If you just want to change something that you entered, the best way to do that is to go back to the interview screen where you entered it and change your entry. You do not use an override to change something that you entered.
If you need further help you have to tell us whether you are using TurboTax Online or the desktop TurboTax software. With TurboTax Online you sign in to TurboTax on the Internet using your web browser. With the desktop TurboTax software you install the software on your own Windows or Mac computer.