To Upgrade your Subscription: 

Navigate to Organisation Settings (1), to the Subscription & Payment tab (2) and click Upgrade (3).

Select "Change Plan" under the plan you wish to proceed with. 

Enter your credit card details (sample digits below) (1), and press Upgrade Subscription (2).

To Change Payment Method:

Return to the Subscription & Payments tab (1), and select the "Add New" bar under the existing payment method (2).

Fill in your credit card details (1), and press Save (2).