If you are a citizen living abroad, you may still be eligible to vote. Learn about your options: https://www.fvap.gov/citizen-voter/overview.
Send in the Federal Post Card Application to register to vote and request a mail ballot. If your ballot does not arrive by October 13th, you can print out and send in the Federal Write-In Absentee Ballot in place of your ballot. You may be able to email in your application and ballot depending on your state.