Answer
To renew your annual subscription, you will first need to login using your email and account password. If you are unable to login, you should reset your password.
Once you have logged in to the My Account section, simply click View/Renew/Upgrade Subscriptions.
Also, when you upgrade your subscription any existing value is applied to your new subscription. For example:
If you have an existing Enterprise subscription ($1649 per year) and halfway through (six months) you need to go to an Agent-25 subscription ($3299 per year) you would pay $2474.50, I.e. $3299- $824.50 (half of $1649).