Because of the 3DS regulation it's not possible to change the card information while you have an active subscription.
First you need to cancel the subscription here
Click on the small trash icons to delete existing data and then insert the new card information here
Once you have inserted all necessary information you need to go back to this page and activate the subscription.
NOTE: Be aware that subscription cancelation is immediately! Just click once and then wait system to respond. If everything is ok then you'll get green notification. If there was some kind of error you'll get red notification with written explanation.