To place or remove roads, you need to first go into your Deco Mode, using the icon with the four arrows in the bottom left of the game's interface. After you've entered Deco Mode, tap the icon in the upper left with a plus and a minus sign on it. Once you've done that, all you need to do is tap on an area to either add or remove a piece of road.

Please note that "Deco Mode" is not to be confused with the "Deco" section of the Shop.