https://techcommunity.microsoft.com/t5/Yammer-Blog/Edit-Post-is-Here/ba-p/61461
We are pleased to announce the launch of Edit post, one of our most frequently requested features! Starting today, Yammer users will be able to make changes to the text in a post or in an announcement after it has been published. After a post has been edited there will be a tag indicating that it has been edited, and users will be able to see the version history by clicking on that tag.