Aircraft registrations expire every 3 years and need to be renewed. The easiest way to renew is to follow the instructions included in the letter the FAA sends you 6 months before expiration. A similar letter is sent a month letter. Both of these letters include a personalized registration code that you can use to register online, which is the easiest way. If you do not have that code you can also renew by sending a paper application to the FAA. In either case, it’s recommended that you submit the registration 5 months before expiration to provide time to resolve any issues. If you allow your expiration to expire then your plane is grounded until you get that resolved.