When you want to upgrade your subscription, you need to go to the license tab and select the subscription level you want and make a purchase at the new subscription level. If you currently are on a monthly subscription, you need to cancel the subscription to be able to select a new one.
If you are purchasing a new yearly subscription:
The time you have remaining on your subscription will be prorated and added to the subscription you purchased. The license tab will tell you when your account will expire so you can see your prorated expiration date.
For example, if you upgrade from professional to elite, your time remaining on your account will be prorated and a year will be added to it with your purchase.
If you are purchasing a new monthly subscription:
Your time remaining will be prorated like when purchasing a yearly subscription, but your monthly payment won't take effect till about 14 days prior to your new expiration. The license tab will provide you details about when your account will start to be charged.