You access the feature by long-pressing the message you want to edit and selecting “edit.” Messages are editable for up to 15 minutes, so you’re not going to be able to amend any older mistakes. Edited messages will display a small “edited” notice next to their timestamp to let chat participants know they’ve been changed. It’s a very similar approach to the one Apple used for iMessage when it added its version of the feature last year.
WhatsApp has long offered users the option of deleting messages, so technically, it’s already possible to fix typos by deleting the offending message and resending it. But deleting a message replaces it with a “this message was deleted” notice, which can look messy. Simply editing a message should be much neater.