Why am I getting a pop up message on my iPhone to update my Apple ID?

You're welcome Joseph.

Well, it's difficult to tell you remotely (perhaps if I could see a screenshot of that message....)

Going to details, it might be also a "scam or phishing" attempt coming from a rogue page in Internet or from a rogue calendar (read this article:-->Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support). NEVER answer to those messages, and never give any personal info to anyone.

I would suggest you two more things:

Clear your Safari cache and cookies, using this simple procedure:--> Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support

and

Check your calendar accounts in Settings>Calendar>Accounts. If you see a suspicious calendar (you accidendally subscribed to) delete that account.

After performing those two steps, restart your iPhone and test it again.

Regards

Giulio

Check your Apple ID info in Settings>[your name], and check the first 3 options.

Regards

Giulio