Most all laptops have two ram slots and the maximum amount of ram is usually not very high, the laptop that you have states that the maximum amount of ram is 8gb and since it comes with 6gb most likely you have two sticks of 3gb. If you want to upgrade to 8gb then you will have to get two sticks of 4gb and take out the ram that you have. Before buying anything you can check out what you have by removing the cover on the bottom of the laptop that covers the ram slots, it should be clearly marked and it's usually a square piece about 3" or 4" square. The slots will be laying down and there will be a latch to release them so that the ram sticks can be removed.
I would question why you want to upgrade from 6gb to 8gb as a 2gb increase will no make much of a performance increase. You would also be buying a new 8gb set of ram.