First things first: sign into your Yahoo! Mail account. Then:
Note: Checking this box means that you intend for mail to stay in your external account AND in your Yahoo! account. If you do not check this box, the mail received from your external mail account to your Yahoo! Mail account will be deleted from your external mail account.
Note: Checking this box means that your old emails will stay in your external mail account and only the new emails received to your external mail address will be sent to your Yahoo! account. If you do not check this box, all old and new emails to your external mail account will be sent to your Yahoo! Mail account.
Note: For security reasons, your password will not appear. If you need to type a new password, just type it in the space provided.