I received an email from iCloud asking me to update my payment details. Is this a scam?

Hard to say just based on what you say here.

Take a look at these articles

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

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

But my best advice would be that even if you think it is real, don't use any links in the email.

To check if you need more cloud storage do this:

Manage your iCloud storage - Apple Support

if you need to update payment method do this:

Change, add, or remove Apple ID payment methods - Apple Support

That way even if you get fooled by a scam email, you have not given any information to the scammers

Hope this helps!