If you are a registered overseas voter returning to the United States prior to an election in which you wish to vote, you need to inform your local election official in the U.S. of the change. The election official to contact is the one in the voting district of your last U.S. residence address. You can look up your local election official's contact information in the Election Official Directory under the “Election Official Contact Details” tab.
You should also complete a domestic voter registration form (as well as a new domestic ballot request form if you wish to vote by mail), and submit it to the local election official in the area where you are now living in the United States. To generate your state-specific voter registration form, you can go to the U.S. Vote Foundation website.