It’s not at all straightforward and this is because Github recently changed the way that downloads works.
Go to http://github.com/osCommerce/oscommerce2
Click the grey (blue when you mouseover it) button;
Up pops the download box;
Click the [Download .zip] button on the popup.
Save it on your hard drive
Unzip it, install it, play with it. Let us know what you think of it!
The .zip that you download will be named 2.2rc2a.zip or such. Don’t know why this is – but it will be 2.3.