You can change the default account for sending messages in the “Writing email” section of the Settings menu.

Hover over the Gear button and select Settings. Select the “Accounts” option and then click your Yahoo! account. Select your new address from the “Reply-to address” drop-down menu.

Hover over the Gear button and select Settings. Select the “Accounts” option and then click your Yahoo! account. Click the “Delete” link next to your extra email address. Confirm that you want to delete the account.