How do you copy video game

if by copy, you mean copy the files of the video game, it is quite simple. but be warned, many companies will pursue a copyright infringement lawsuit if they know you are doing this as you do not have the rights to legally reproduce or replicate their IP.
onwards then. If it is a computer game, one you have downloaded not an online one. then simply look it up in your computer files. Find the file and right click it for the menu. From there you should be able to click copy to duplicate the files, then paste anywhere to create duplicate files.

Its a bit trickier for a console, and i only own an XBOX360 so i would not know if this works for a ps3. get a usb (At least one gig of space and DONT USE ONE WITH IMPORTANT INFORMATION ON IT, IT WILL BE DELETED!) and jack it into the xbox. go to system settings>memory. from there select the usb and click on configure. now that the usb is now usable, open up your hard drive for your xbox and go to the files you want copied. select move and move them into your usb. now simply take the usb and put it in your computer. From there you can open the usb up and simply copy the files just like how you did for a pc game.