I got an email from apple support that my apple ID will be permanently locked if I dont' update my payment method. How do I know if this is a legitimate email?

I can confidently reply, that Apple will never threaten to "permanently" lock your Apple ID. Do not enter any information. You are experiencing Phishing:

1. Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
2. Identify legitimate emails from the App Store or iTunes Store - Apple Support

It's a scam.

About Email and Other Scams

Apple always addresses you by your name or the name they have on file for you.  The Email will be from apple.com or itunes.com. Email addresses can be spoofed. You can go to Mail > View > Message > Show all Headers to see more. Apple Emails will never contain an attachment. Apple will never request personal information such as Social Security numbers.

An exception to the above is when you order something from the Apple Store. The receipt will be addressed to Dear Apple Customer.

Avoid Scams

1. Identify and report phishing emails and other suspicious messages
2. Annual Reminder- Watch out for email Phishing Scams
3. Identifying fraudulent "phishing" email
4. Beware of Browser Pop-Up Tech Support, Phishing Scams
5. Phishing & Other Suspicious Emails
6. Avoid phishing emails, fake 'virus' alerts, phony support calls, & other scams
7. Phony "tech support" - "ransomware" popups and ... | Communities

Report phishing email to abuse@apple.com.