You can edit and re-share a post by tapping on the post from the calendar. This will open the "Post Details" where you can see engagements with the post by network. In the upper right corner is a pencil/edit icon that will create a copy of the original post for you to edit and re-share. 

You'll end up in the "customize" step where you can double tap images or text to edit. 

Note: Editing and re-sharing your post does not edit past posts already on social media. To change past posts, you need to delete the post on the social media site and repost the edited version.