My Yahoo email is synched with my T-Mobile phone. A message keeps asking

[MarilynOwens]

to update password or its wrong.? I have taken the phone to T-Mobile and they deleted and reentered my Yahoo email, but after several days this message keeps coming up: "Update your password for winmo.imap.mail.yahoo.com:993.1 and tap save." I continually put in my Yahoo password but it still will ask me again later.