If you are from a state outside Missouri, you may still have time to register and vote absentee in the upcoming election. Check the rules and deadlines NOW. Google has a widget for checking your deadlines and procedures. There is nothing stopping you from registering to vote in another state where you are qualified to vote, and changing your registration to Boone County, MO after the next election has passed.
The online form linked below will also work if you are registering to vote in another Missouri county. It is a statewide form, and as long as all of your information is received by the Boone County Clerk by close of business on October 12, it will count as an on-time Missouri registration.
If you do opt for an absentee ballot, make sure you know the other jurisdiction's procedures and deadlines, and allow ample extra time to allow for unforeseen glitches, if you want your vote to count. See the "Every State Votes Differently" section of this guide for a few examples of state-by-state differences. Check the procedures and deadlines now to ensure your vote will count.