I believe you meant the Lenovo Z580 since there is no such model as the Z850.
http
/www.lenovo.com/products/us/laptop/ideapad/z-series/
While Lenovo's website states a nVidia GT 640m is an optional upgrade ("up to nVidia GT 640m), when you select a laptop to configure you actually do not get an option to upgrade to a nVidia GT 640m. Lenovo may decided to eliminate all optional upgrades for the Z580 laptop. You can't even upgrade the RAM or hard drive.
http
/www.lenovo.com/products/us/laptop/ideapad/z-series/z580/
Go back to the website where you bought it from can review the listed specs. If it states the nVidia GT 640m is included and is not an upgrade option, then you were shipped the wrong laptop.