Under the IRS e-filing rules, if the IRS rejects an e-filed return, the IRS employs a 10-day lookback rule that will consider the return filed on the original submission date if all issues are corrected within 10 calendar days. The date is not extended regardless of weekends, holidays or the end of the year cutoff. Thus, if you entered in the wrong EIN, you should be able to cure the return within this 10-day period without filing an amended form 941-X.  However, if you do not correct the return within this 10-day window, then IRonMaN and the others are correct, you would need to file an amended form 941-X by paper (as the IRS does not accept this form electronically). This comment isn't so much for you (as 10 days is long past), but might help the next person searching for an answer.