In order to edit or re-share a post on the web app, first tap on the post from the calendar. This should pull up a detailed view of the post where you can see who is engaging with your post. You'll then navigate to your post, click into it then select "Edit" at the top left.
To edit or re-share a post on the mobile app, tap on the post from the calendar (located on the "My Business" screen under "Schedule.") This will open the "Post Details" where you can see engagements with the post by network. In the lower left corner is a "duplicate" 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.