Step 1. Go to: 

Step 2. On the upper right corner of the homepage click on "Login/Register"

Step 3. Enter the E-mail address associated with your subscription. 

Step 4. Check your email for the verification code (Should look like the screenshot below):

Step 5. Once the verification code has been entered, you should now be logged in. 

Step 6. Click on the arrow to the right of your E-mail address. A drop-down menu will show up.

Step 7. Your active subscriptions will automatically show under the "Subscriptions Tab" click "Manage Subscription" on the one that you'd like to add/change the payment method for.

Step 8. A smaller window will pop up. You will see an option for "Payment Methods". Simply click this and enter your card or Paypal account information! You're done!