I have the original iPad and at some point my iPad told me it needed to upgrade to ios 4.2. So how do I do this ?

You need to have the latest iTunes installed on your computer and you must update by connecting the iPad to the computer, launch iTunes, select the iPad on the left side under devices and then click on the Check for Update button in the iTunes Summary Pane on the right side.

It is described for you here as well.

http://support.apple.com/kb/HT4623

And actually the current version of iOS is 5.1.1. 4.2 was replaced some time ago.