You can't just take a published game and edit it, I'm afraid. Even if it were technically possible (which it's not), there are are legal restrictions in doing so.
Instead, you need to create your own game from scratch, using whatever bits you like about that game, together with your proposed improvements. To learn how to use Construct2 to create games, read the manual on this site and open up the supplied examples - the description of the game sounds perfectly doable but, as you're new to programming, I suggest you start small - start with the basics of drawing a character to the screen and moving it around etc.