Sounds like they only created a 6gb partition on the drive.
actually increasing the size of this partition would require specialist software, and could destroy the partition so it would need to be reinstalled again. creating a second partition to use the left over space on the drive would be very easy, but a 6Gb system partition is going to cause you problems in the near future as it will quickly run out of space.
Have a look here for a tool to resize the partition.
http://www.partition-tool.com/easeus-partition -manager/help/resizing-and-moving-partition.ht m
Make sure that there is nothing on the laptop of any importance before doing it though, as I said there is always a risk when resizing that you lose everything!