Open Facebook or Messenger
Go to the chat containing the message
Press and hold the message
Select Remove or Delete
Choose Unsend for Everyone if available
Confirm the action
Repeat for any other messages you want to delete
Open Facebook or Messenger
Go to the chat containing the message
Press and hold the message
Select Remove or Delete
Choose Unsend for Everyone if available
Confirm the action
Repeat for any other messages you want to delete