If you have not yet transmitted (E-filed) your return then you can do this by going to the "Federal Taxes" tab, click on "Wages & Income", then click "edit/add" next to your W-2.
However, if you have already submitted your return, you will
not be able to go back into your account and make changes. If the return has
already been e-filed, any changes you make to it now, the IRS will not be
notified of the changes.
To correctly make
changes to your return you must amend
it. The amended process will not be available until mid-February. At that
time click here and
follow the instructions on how to amend a 2016 return.