This article provides instructions on how to delete messages from Messenger via the Facebook website and via the Messenger app.
How to delete Facebook Messenger messages?
Messenger keeps all of your messages in your inbox until you decide to delete them. You can delete individual messages and entire conversations to clean up your inbox. The following instructions show you how to delete messages on Facebook.com.
Select the Messenger button in the top right corner.
Select Show all in Messenger at the bottom of the Messenger window.
