Edit an existing signature in Yahoo mail is quite an easy task; one just needs to get through the account “Settings” from the menu and under “Writing email” in the left pane, make sure you’ve swiped right the toggle button. Then edit your signature in the text box. This is how you can figure out how to change a signature in Yahoo mail account.

Add, edit, or disable your signature in Yahoo Mail
Click the Settings Menu icon. | select More Settings.
Click Writing email.
Click the Toggle button. to enable or disable a signature for your email address.
Enter or edit your signature in the text box.

Hope This Works,
Peter