I have those same shifter / brakes on one of my bikes. Just remove the cap (careful no to lose those small screws), and push the old shifter cable through. Make sure the shifter is in the lowest (most relaxed) setting...chain on the smallest sprockets. Installation is reverse of removal.
Watch a few youtube videos for tips on installing the cables. There are several tips that make things easier, such as making sure you have the fine-adjustment knobs in the correct position prior to attaching the cables to the brakes/derailleurs.