I've Got Far Too Many Emails In My Inbox On Yahoo! Mail. Can I Delete Them All At Once?

Yes. Sign in to your Yahoo! Mail. Go to your inbox and tick the box next to the emails that you wish to delete and click 'Delete'. This will delete all selected emails. If you are receiving a lot of emails each day and you don't have the time to read them but wish to keep them, then just tick the box next to the emails, click 'Mark' and then click 'Mark As Read'.

This will mark the emails as though you have read them, so that it is easier for you to manage your inbox and not miss important emails because you have too many unread ones.

ya!  First you sign in your mail then select all messages by check mark the button which is in header and left side of sender, so your all messages are selected now you have to click "delete" button so you can delete all messages at once.