If you are using online TurboTax you do not have forms mode or direct access to the forms. Forms mode is a feature of the desktop download software. With online, you enter your information via the interview screens only.
Why do you think you need to edit the actual forms?
If you are using the TurboTax online web-based editions you do not have access to any forms, schedules or worksheets. That option is only available using the TurboTax desktop editions using Forms mode.
Are you in Premium? That is an online version and is different than the Desktop installed program. In Desktop there is a Premier version- different name. There are many differences between Online and Desktop. The Forms mode is only available in the Desktop program.
If you can clarify what you are trying to enter, we'll try to help.
Are you using TurboTax Online or the Desktop version (which has FORMS access)?
Here's some info on Transferring a Prior Year Return to get started.