The main posts page in the WordPress admin has a nice quick edit feature that allows you to quickly update, among other things, the date the post has been published. Hover over the post name and you’ll find the quick edit button. Change to the desired date and then hit update.
Publish Pane Edit
While the quick edit is certainly quicker, there’s also an option to change the post date while editing a post. Find the Publish pane on the right and you’ll see ‘Published on’ as well as an edit link. Clicking that link will allow a quick edit of the date. Hit OK and the next time you update the post the date will also be updated.