It’s a feature that users have been waiting for a while now, because WhatsApp is preparing it for a while. They just started to roll out the updates, and you’ll be able delete/recall messages after they’ve been sent.
To access the delete functionality, you just have to tap and hold on the respective message. You can choose 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.
Of course it’s not a perfect thing. First, it’s going to work if you choose to delete in maximum seven minutes after you sent it. Then, deleting a message gives no guarantee that the message wasn’t seen by the recipient. Also, there is no notification if deleting for everyone was successful or not.
As the update is just rolling out it may take time to get to everybody.