By default, the only clock widget that comes with the GN is that Movado-looking clock, but fret not there are hundreds if not thousands of alternatives in the market, quite a few are free.
To remove the current widget from your screen, go to the screen where it is, long press on it, and drag it to the trashcan or X that appears at the top of the screen and release it on it.
To add a widget to your screen: Pull up your app drawer, near the top left corner click widgets (you can also get here by scrolling to the right past all of your apps), find the widget you want to add, and long press/drag where you want it on your screen.
[video=youtube;LPVVqk5RIpI]http://www.youtube.com/watch?v=LPVVqk5RIpI[/video] <-- quick Google search showed that. Not really a how to, but shows what I am talking about.
