How to Delete All Unread Messages in Gmail at once

Visit Gmail with a browser. Gmail actually has a fairly advanced query language: Using advanced search

Follow these steps:

1. Search with this query:
   

   (in:inbox) and (is:unread)

2. Select all messages:
   
   
   
3. At the top of your messages, click on "All __ conversations on this page are selected. Select all conversations that match this search."
4. Click on the delete icon.