Retail MSDN Subscriptions (purchased directly from Microsoft or from a retail reseller) are renewed annually and can be renewed only starting 90 days before expiration.
If you purchased your MSDN subscription through a Microsoft Volume Licensing reseller, please contact your reseller for details on renewing.
If you were assigned an MSDN subscription through your company, you need to connect with your MSDN administrator to renew your subscription. This applies for MPN, BizSpark and Volume Licensing subscriptions.
Retail MSDN subscriptions can be upgraded by purchasing the higher-level MSDN subscription desired at its "new" price, with the exception of MSDN OS which can renew to Visual Studio Professional with MSDN at the “renew” price. Compare subscriptions to determine if upgrading is the right path for you.
It’s also allowed to renew a subscription that was purchased in the retail channel onto a Volume Licensing agreement, then to upgrade to a higher subscription level by purchasing a Step-up License.