Click on the Login button and enter your credentials
Click on the pricing link at the top
Under the new subscription level that you want, last row of the pricing table has a button, click that button and it will redirect you to PayPal to complete the setup of the new subscription
Once successfully done, you now have 2 active subscriptions – old and new
So, you need to cancel the old subscription
To cancel old subscription, login to your PayPal a/c, go to pre-approved payments page and cancel the old subscription