Did your company provide you with an MSDN Subscription purchased through Volume Licensing? If so, contact your MSDN Administrator to renew or upgrade your subscription.
Retail MSDN Subscriptions (purchased directly from Microsoft or from a retail reseller) are renewed annually and can be renewed anytime during the subscription. Depending on your subscription level, you’re eligible for a variety of renewal options.
* You’ll receive Visual Studio Enterprise with MSDN with this renewal purchase.
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.