You can renew starting 30 days before the expiration date of your existing membership or any time after it expires.
If your Apple Developer Program membership expires, your apps will no longer be available for download and you will not be able to submit new apps or updates. You will lose access to pre-release software, Certificates, Identifiers & Profiles, and Technical Support Incidents. However, your apps will still function for users who have already installed or downloaded them, and you will still have access to iTunes Connect and free development resources.
If your Apple Developer Enterprise Program membership expires, your apps will no longer be available for download and will no longer function for those who have already installed or downloaded them. You will still have access to free development resources.