Why do I keep getting e-mails that say I have to upgrade or you will stop my mail?

Answer 1

They're all scams. You do not upgrade your email account at all. Scam messages that request you to "upgrade" your email account must be ignored and deleted immediately. Never click on the links or attachments and give your personal information to the scammers or you can risk your account being hacked. Remember Yahoo would never send you unsolicited emails asking for your personal information.

Answer 2

Scammers are sending millions of those emails to phish your account details.

Answer 3

That is a phishing scam. Ignore and delete it. This is how to identify legitimate Yahoo websites, requests, and communications: https://help.yahoo.com/kb/SLN2091.html

Answer 4

Those are probably scams, and not from Yahoo.  Don't "upgrade."