I'll show you how to do it, since you're an admin and because it's really easy. The logo is located at File:Wiki-wordmark.png. Go there and click on the File History tab. Find the version of the image that you want to switch to, in this case the second one down (14:36, January 9, 2014), and simply click the "revert" link beside it. This works for any image, by the way, not just the logo.