WhatsApp is getting a new feature, and a long requested one at that. The feature in question lets you delete/recall messages after they've been sent. To access the delete functionality, just tap and hold.
Then you get to choose whether to delete messages for everyone or just for yourself.
If you go for "delete for everyone," the message will get deleted from recipients' chat window as well, replaced by "You deleted this message" in your window and "This message was deleted" for others.
There are, some points worth mentioning here. Firstly, 'delete for everyone' only works for up to seven minutes after sending the message. Secondly, deleting a message in no way ensures that the message wasn't seen by the recipient. And finally, you won't get any notification if deleting for everyone was not successful.
The feature is currently rolling out, so it's possible that you won't be able to use it right away.