Wait until the original return has been fully processed. Then you can amend. You will go to Federal>Other Tax Situations>Other Tax Forms>Amend a Return
when you start the app you should see a screen that at the bottom left says amend a filed return - click on it to start. in some cases based on screen resolution you might not see it so you'll have to scroll down. after you click on amend you should see another screen with a blue icon having "amend" at the bottom. double click on that.
then follow the prompts. you'll come to another screen saying "what do you need to change" next to W-2 you'll see "update". click on that. then click on the "add another w-2" link