If you own an aircraft that was registered in the month of February of any year before October 1, 2010, your aircraft must be re-registered by December 31, 2013. You should submit your re-registration application as soon as possible since you won’t be able to operate your aircraft legally after December 31, 2013 unless you have received your new registration certificate. Online re-registrations will be accepted until October 31, 2013. After that, you will need to submit your re-registration application to the FAA via mail. Online re-registration applications may be submitted here by entering the aircraft’s N-number and re-registration code to begin the process, or you may submit a written re-registration application to the FAA via mail.
For a discussion of the FAA’s re-registration requirements, please read my article Understanding The FAA’s New Aircraft Re-Registration And Renewal Requirements. Additionally, the FAA has a Frequently Asked Questions page regarding re-registration which you can view here.