iCloud says my payment method has expired and to update payment

Check if there's a problem with your current payment method.

Read this document about how to view your current payment types and add Apple ID payment methods on an Apple mobile device, a Mac, or a PC --> Add a payment method to your Apple ID - Apple Support

If an item appears in red then it is no longer working and you either need to replace it or get it working again.

You also don't say how you are being notified of this. Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. Here are some guidelines:

Identifying legitimate emails from the App Store and iTunes Store - Identify legitimate emails from the App Store or iTunes Store - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams - Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Avoid scams when you use Apple Cash --> Avoid scams when you use Apple Cash - Apple Support

Apple 'How to identify, avoid, and report phishing' video--> https://youtu.be/SR3Z3fXXjfw

If you are uncertain about a message and a resource provided in that message, do not click on any links in the message. Try to independently verify the resource by going to a support page on apple.com and use that to contact Apple or to verify with the service the message is telling you to use.