You can go to the yahoo-help to learnt how to reset your password.
You can get a new password, a Yahoo! ID reminder, or both. If you forgot your Yahoo! ID or password, go to the Sign-in Problems page.
If you forgot your Yahoo! ID, you need to provide three pieces of information so we can confirm your identity:
Your ZIP or postal code (at the time you created your Yahoo! account or when you last updated it)
Your alternate email address -Note: You must have access to this email address
Country code (at the time you created your Yahoo! account or when you last updated it)
Be sure to provide the same information you gave the last time you registered for a new Yahoo! ID or password or the last time you updated your account.
If you forgot your password, you can create a new one. But first you'll have to provide the following information so we can confirm your identity:
Your Yahoo! ID
Alternate email address, mobile phone number, or the answers to your secret questions
A Reset Your Password link will be sent to your alternate email address. Click the link and enter your new password in the Reset your password page.
If you entered your secret question incorrectly or forgot what you originally entered, for security reasons, we cannot tell you what your Yahoo! ID is or let you create a new password
if you forgot yahoo mail password and secret question
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