Setting Primary/Backup payment method
You can change the primary method as well as add/set a backup payment method to your account.
The Card/PayPal account you add when you sign up with a premium plan is set as the primary payment method. To change a primary method, you'll need to add another Card/PayPal account and set it as the primary payment source as shown in the GIF below.
Changing the Primary payment method to a backup payment method
You could also set a backup payment method using the same steps as above, but selecting the payment method as Backup instead of Primary. This way, you will then have two payment methods, a Primary and a Backup to keep your subscription active and uninterrupted.