Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Sorry to hear you're having trouble.
Please log in to https://account.microsoft.com/services and find the subscription you're looking to change. Click on Manage on the subscription and look for the payment method. You should see an option to Change linked underneath.
If you have any issues doing this then please start an online chat with support via https://support.microsoft.com and click on Contact Us then follow the instructions until you reach a person.