Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
I'm being honest with you. Unfortunately, there is NO easy way to recover the overwritten files. It doesn't matter whether it is an excel
file, movie, word etc. Once a file is overwritten, the content is usually replaced with new one. Don't you have any backup, like Time Machine ?
If a file is deleted, then you can easily recover it with some third party recovery applications. But for overwritten files, these applications might not work.
You should check the Office AutoRecovery folder of your Mac. Goto Macintosh HD (your hard disk)/Users/home folder/Documents/Microsoft Userdata/Office 20x_ AutoRecovery/ folder
Open all files there and see if you can find that file.
If you can't find it there, then please try some third party recovery application like this, iSkysoft --> http://www.iskysoft.com/computer-data-recovery/recover-overwritten-files-mac.html
Recovery applications are very costly, so please try the trial version before buying it.
If recovery application doesn't find it, then you will not be able to recover it unfortunately.
I know this might not be answer you want to hear, so please don't rate me negatively if you are unable to recover it. You don't have to rate anything if you are not satisfied.
If you have any questions or would like any additional information, please don't hesitate to ask.