1. Login to your account.

2. Click on My Account in your menu

3. Click on "My Subscription"

4. Click on the Number of your active Subscription

5. Click on the Upgrade / Downgrade button

6. Choose your product and checkout.

When upgrading your subscription you will get a prorated discount based on the time left of your current subscription. When downgrading, you'll receive additional time credited to your new subscription based on how much time you had left.