From most Yahoo mobile apps:

1. Tap the Menu icon.
2. If using the Yahoo Mail app, tap Manage Accounts.
3. Tap Account info.
4. Tap Security settings.
5. Enter your security code.
6. Tap Change password.
7. Tap I would rather change my password.
8. Enter the new password and its confirmation and tap Continue.

Or Dial Our Yahoo Mail Customer Service Number  |Yahoo Customer Service Number