mcdevito75 here, Look for a small watch repair shop in your area, they may have to contact Guess watch for a new back cover, unless you find one say on ebay.
Takes a special tool to remove back off watch. You can buy the entire watch tool kit online for about $65. OR, you can send to Guess (best option as it will keep the waterproofness intact) for about $11.50 (see link)http://www.guesswatches.com/pdfs/WarrantyBooklet_GuessWatches.pdf