first electornic filed return rejected; fixed return demanded more tax due; already sent a check for lower amount. what do I do

I would send a check for the additional amount due.

It might be easiest to set up an account with them.

IRS Individual Online Account  

• Taxpayers with a Social Security number or an Individual Taxpayer Identification Number can create or access their IRS Individual Online Account to get information they need to file their return.
• With an IRS Individual Online Account, people can:
• View balance owed, payment history and schedule payments.
• Cancel scheduled payments.
• Get transcripts.
• View or create payment plans.
• See digital copies of some IRS notices.
• View key data from their most recently filed tax return, including adjusted gross income.
• Validate bank accounts and save multiple accounts, eliminating the need to re-enter bank account information every time they make a payment.
• View, approve and electronically sign power of attorney and tax information authorizations from their tax professional.