Apple Pay payment information

I received an email stating the following:

On 12/17/2021 you are scheduled to be charged $3.99 for iCloud+ with 200 GB GB of storage, but there is a problem with your payment information.Please update your payment information as soon as possible. If we cannot successfully charge your account, it will be downgraded to the free 5 GB plan and your service may be affected.

I haven’t changed anything. Why is this happening and what do I do?

[Re-Titled by Moderator]