Open Facebook or Messenger
Go to the chat containing the message
Press and hold the message
Select Delete
Confirm Delete for You
To delete an entire conversation, open the chat list
Press and hold the conversation
Select Delete
Confirm deletion
On desktop, open Messenger
Click the three dots next to the message or conversation
Select Delete
Confirm the action
To unsend a message you sent, press and hold the message
Select Remove for Everyone
Confirm removal if available