Hello. My name is, Mike. I'd like to try and assist you with your issue. Please follow the instructions below:
In Yahoo Mail
Step 1: Click on the gear icon in the top right corner, then select Settings
Step 2: Go to the Account tab, then under the Yahoo account section, click on the Edit button.
Step 3: Enter a new name in the Sending Name box, then click Save. Click Save again to save your new settings.