That depends on your model of iPAQ and country of origin. Each model has its own version of the upgrade. They are available only from HP and not all models have upgrades available. The link for the UK, for example, is:
http://www.ipaqchoice.com/generic2.aspx?page=PPCUPGRADE
For the US they can be found via:
www.shopping.hp.com - search under handhleds and accessories.