You accidentally deleted a message on your Facebook account and now would like to return? Follow the below procedures and you should return the message that you’ve deleted.
– Log in to your Facebook account 
– Click on the message (on the left menu bar)
– Press the More and mark Archived (Archived)
– Now should be showed on the left all deleted messages.
– Select the message you want to restore> then press Unarchive
Note!
If you delete a message from the “archive” keep in mind that these are copies of the message and if it is deleted, you will not be able more to bring them back.
If you permanently deleted a message on Facebook, also they will not be able to return.