The EIN is one of the first things requested to be entered on the W2 form entry process.
If you have already entered the W2, then you can select the "Wages & Income" button which will take you to the list of available W2 which you could then select and edit the appropriate one to add the changes, including the EIN directly to the existing file.
If there are no W2s, the app will ask for some information about who it is for, then it will ask for the EIN as one of the first things.
If the W2 you wish to modify is not in the list, you can add a new one using the EIN that you have and if the other shows up somehow, you can delete it when it does appear.
