You can edit and/or re-share a previously created post by tapping on the post from your calendar or by selecting a post from the recent feed listed under your "Weekly Engagements" graph. This will open a detailed view of your post where you can see who is engaging with the post. 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.

Keep in mind, editing and re-sharing your post does not edit posts that have already been shared to social media. If you want to change a post you've already shared, you'll need to delete the original post from social media and re-post in order to see the changes.