just to confirm - you want to delete all emails from yahoo email a/c?
If so, then when you login into your yahoo a/c, go to your inbox , above the first email you will see headers like:
"from" .... "subject" .... "date".... etc etc
on the left hand side of that title bar, there is a tick box... click in that box & then a tick mark should appear in all of the emails... then click delete.
or
within the inbox, click on "Actions" - "Select all email"
then click on the delete button.
If you need more details, just let me know... should be similar for most online emails a/cs.