To do this is very simple. When SketchUp opens, it shows a welcome panel. If you are running the demo, this will tell you how many days you have left or that it has expired. If you have licensed SketchUp already, the panel shows tips and options.
To add or change your licence details, simply click on the 'Licence' section. It's that simple. Provided you have an internet connection and your licence isn't in use on another machine, SketchUp will activate your licence.