If it's connected to the Net go to http://www.java.com and click the big red Download Java button. Click to Save and pop it on the desktop where you can see it and delete it later. When the download completes, double click the desktop icon and let it install. When it completes, you'll see a web page inviting you to Verify Java - click on that and let it finish the process.
All that will put an entry into your configuration to check regularly for Java updates automatically so you'll always have the right version.