just got a message to change my apple id password. How do I know this is legit

It is probably a scam or phishing attempt.

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 iTunes Store - http://support.apple.com/HT201679 - lacking in specifics, the next link is better:

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - https://support.apple.com/HT204759

- Apple e-mails address you by your real name, not something like "Dear Customer", "Dear Client", or an e-mail address*.

- Apple e-mails originate from @apple.com or @itunes.com but it is possible to spoof a sender address.

- Set your email to display Show Headers or Show Original to view Received From. Apple emails originate from IP addresses starting with "17.".

- Mouse-over links to see if they direct to real Apple web sites. Do not click on them as this just tells the spammer they have a working e-mail address in their database.

- Phishing emails may include account suspension or similar threats in order to panic you into clicking on a link without thinking. They may report a fake purchase in order to infuriate you into rashly clicking on a false link to report a problem. Mar 2018 post by Niel https://discussions.apple.com/message/33129140#message33129140 - "Emails saying that your Apple ID has been locked or disabled are always phishing. If one actually gets disabled, its owner will be told when they try logging into it instead of through email."

- Apple will not ask for personal information in an e-mail and never for a social security number.

- Scams may have bad grammar or spelling mistakes.

- Apple will not phone you unless it is a response to a request from you to have them call you.

* Exception: https://discussions.apple.com/message/33701414#message33701414

Forward attempts as an attachment (in MacOS Mail use the paperclip icon) to: reportphishing@apple.com then delete it.

If this is with regard to a supposed purchase, this Apple article has relevant information and web links for checking if you really have made a purchase or paid for a subscription: If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - Does not mention purchases made under Family Sharing which might be charged to the organizer's card but will not appear under the organizer's purchase history or subscriptions. Ask family members about those or check your receipts. --> https://support.apple.com/HT201382