Go edit the post in the post editor.
Normally the author section isn't displayed on the post editor. If you do not see it, click the Screen Options tab in the upper right corner, then check the box labeled Author. The Author drop-down list will now appear towards the bottom of the post editor page where you can change the author.
On the right side of the post editor is a section titled Publish, and the last line on that section will read Published on: followed by a date. To change that date, click the Edit link to the right, change the date, and click the OK button.
Once all of your changes are in place, click the Update button.