First, be very careful about emails. 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 email attempts as an attachment (in MacOS Mail use the paperclip icon) to: reportphishing@apple.com then delete it.
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If you are sure this was from Apple and wish to proceed with upgrading then make sure you have a valid payment method active on your account: This document explains how to 'Change, add, or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266
Now proceed to upgrade your iCloud storage. Click on this link to discover how to "Upgrade your iCloud storage plan" --> https://support.apple.com/HT201318