I’m in a similar situation (new credit card, seven months left of the subscription) and I successfully managed to update just now. Go to GCN+ web page, log in, choose ”Manage account” and from there you can change payment method.
Not on my browser or in my App. I’ve had three emails from their support and they tell me it is not possible. I’m looking at Microsoft Edge as we speak but I’ve also tried others. Maybe it just U.K. but they’ve removed all settings for payment details.
Exhibit B. I rest my case.