In the conversation where you'd like to delete a message:
- Long press on the message.
- Tap Delete.
- Tap Delete for me, or Delete for everyone.
Delete for everyone is only possible for messages that you've sent. The message will be deleted on all of your devices and your contacts' devices. Your contact will see the Trash icon in place of the deleted message.
Delete for everyone is supported from version iOS 2.14, Android 2.14.275. Deletion will only take effect when your contacts' devices come online.