How can I select all unread messages on the new version of Yahoo Mail?

Just type is:unread in the search bar and click on Search mail. Then if you want to delete them all at once, click on Select all and Delete.

I delete my messages as follows: Select page you want to delete (in my case I view 200 messages per page), select sort by date, all of the messages on that page will be selected, then select delete, all messages on that page will be deleted. Repeat process for each page you want to delete.

To delete items showing in INBOX as unread Using Yahoo 7 - If you right click on INBOX a drop down box will appear with Mark as Read - Left click on this box and it's done. They are deleted from your 'unread' without being seen by you. If you want to read them don't do this because you have no way of identifying which ones they were. They are now listed as 'read.'

Just click purple box next to From header.

Use Advanced Filter, then pick out all messages in inbox with a common keyword, then "Select All" or Ctrl+A, delete, you can do more than 100k messages at once but it's slow.

Type is:unread in search box. Select Mark as Read in drop-down menu labeled More

If you need to delete the unread items within a specific folder then you can enter the command in:Foldername is:unread click on the Search button. Select all by clicking the checkbox in the header and click Delete. This will delete all unread messages in the folder. If you want to delete all the mails, regardless whether they are read/unread, just type in:Foldername, search and delete.

In the search box type: is:unread then left-click the box above the emails and select all then hit delete icon. It will delete hundreds at a time to the trash.