Hi!
You can find a guide on how to replace the CMOS battery for this laptop in the service manual that I linked [http://downloads.dell.com/manuals/all-products/esuprt_laptop/esuprt_latitude_laptop/latitude-e4200_service%20manual_en-us.pdf|here, at page 14.]
It is not a standard battery like on your PC, laptops have them wrapped up in some kind of plastic thingie and when replacing them you should replace them with one what's also in the same type of cover. [https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwiZ27P1z_rWAhVhD8AKHYnNBCsQFghWMAM&url=http%3A%2F%2Fwww.ebay.com%2Fitm%2FNEW-Dell-Latitude-E4200-E4300-BACKUP-RTC-Reserve-CMOS-BATTERY-GC02000KG00-%2F112227584710&usg=AOvVaw2TSTJWCUq1CQYy5DoehZFU|You can see here what I'm talking about.]