You can't. The 2 gigabyte they sell you is 2,000,000,000 byte (1000 byte to a kilobyte, 1000 kilobyte to a megabyte and 1000 megabyte to a gigabite) where in fact 2 gigabyte should be 2,147,483,648 byte (1024 byte to a kilobyte, 1024 kilobyte to a megabyte and 1024 megabyte to a gigabite). So when you devide 2,000,000,000 by 1024 three times to get the number of gigabyte, you end up with 1.8626 gigabyte.
However, to avoid confusion, now kilo, mega and giga refers to multiplication of 1000 times (to stick to IS decimal convention) and kibi, mebi and gibi refers to multiplication of 1024 times. So in fact, the company is right to sell you that 2 gigabyte drive, which should be correctly called 1.8626 gibibyte. The computer is just not making a distinction between gigabyte and gibibyte.