Step 1:- On the Search field, type is:unread
(Make sure you are typing the above correctly, or else it will delete all your read messages)
Step 2:- Click the select-all check-box to select all Unread email
Step 3:- Now all the unread emails will be displayed.
Step 4:- A yellow label will be displayed with text "All 100 conversations on this page are selected ." Click on the link "Select all......" .
You get the following message (yellow label). Click Delete.
Click on Inbox to confirm.