How to Delete Messages and Conversation on Facebook Messenger App on Android-2019?
To delete an entire conversation using Messenger for Android, tap and hold your finger down on a conversation, then tap Delete. To delete an entire conversation using Messenger for iOS, swipe left on a conversation, tap More, then tap Delete.
How To Delete Messages On Messenger From Both Sides
https://www.youtube.com/watch?v=a9O6fZyErLs