You can easily renew an existing license key/subscription by navigating to your purchase history. In order to get there you will need to log in. To log in simply click on the Log In button in the top right corner of

Once you are logged in, or if you are logged in already, you will need to navigate to Account > Purchases. 

Then simply click Renew License of the key that you wish to renew.

You will then be taken to the checkout page where you can renew your subscriptions.