To upgrade to paid plan or add certain apps you need to add a valid credit card.
Go to menu > Account > Add credit card.
If you have a paid subscription plan and would like to change your credit card you will need to enter a second card first before being able to remove credit card.
To remove all credit cards you need to uninstall any paid addons and change your subscription plan to free. Once this occurs you will continue to have access till next billing date but be able to remove your credit card.