So to be honest, we are yet to simplify the process of switching plans but here is how you can do this  

Step 1: Turn off Auto Renewal

Step 2: After your subscription has expired (30 days from the date of activation of your subscription), go to Settings.

Step 3: Go to My Account

Step 4: Tap on 'Renew' button

Renew button will take you through subscription options as per your network and device.