Changing your Yahoo password on OS X Mail El Capitan is a straightforward process that ensures your account stays secure. To update your Yahoo password, follow these steps:

1. Open the Mail app on your Mac.
2. Go to 'Mail' in the menu bar and select 'Preferences'.
3. Click on the 'Accounts' tab.
4. Choose your Yahoo account from the list on the left.
5. Click on 'Server Settings'.
6. Under 'Incoming Mail Server', update the password field with your new Yahoo password.
7. For the 'Outgoing Mail Server', repeat the password update.
8. Click 'Save' to apply the changes

Once you've updated your Yahoo password in OS X Mail El Capitan, remember to also update it on any other devices or applications where your Yahoo account is connected.