The tag below your question indicates that you are using TurboTax Online. There is no way to override an entry in TurboTax Online because there is no forms mode.
In the CD/Download TurboTax software, to override an entry in forms mode right-click the entry you want to override, then select Override on the pop-up menu. The value that has been overridden will be displayed in red instead of black. Not all entries can be overridden.
An override can cause the automatic calculations to break in unexpected ways, even in places that you think are not related to the override. An override cancels the accuracy guarantee and also prevents e-filing. You can use an override to experiment, but you should not need an override in a return that you are going to file. If you need an override, you might be trying to make TurboTax do something that it is not designed to do or does not support.
