Open Gmail on your computer.
In the search bar, type "in:all" and press Enter. This will select all emails in your inbox.
Click the checkbox at the top left of the email list to select all emails on the current page.
A message will appear at the top of the email list saying "All 50 conversations on this page are selected. Select all conversations that match this search."
Click on the link "Select all conversations that match this search."
Click on the "Delete" button (trash bin icon) to move all selected emails to the Trash.
To permanently delete the emails, go to the Trash folder on the left sidebar and click on "Empty Trash now.