WhatsApp is working hard on adding new features, especially ones that some of its competitors have had for a long time. Today, it's time for editing.
WhatsApp now supports editing sent messages. All you need to do is long-press the one you want to change, and then select Edit from the menu. This only works in the first 15 minutes after a message is sent, keep that in mind.
Once you've altered a sent message, it will display "Edited" alongside itself when other people see it, so you can't do any stealth shenanigans. The edit history will not be shown, though.
Of course all of these are still going to be end-to-end encrypted. This new feature is already rolling out to WhatsApp users globally, and the rollout should complete "in the coming weeks".