I have an ACNS with two hard disks of 36GB, software version ce590-4.2.1. Is there a way to update its version to a 5.X or even to a 4.2.X?

I already browsed on the software updates page, but can not find a way to update an ACNS starting from the 4.2.1 version.

In fact in the link below reads that "Any ACNS 4.2.x release (except ACNS 4.2.1) to any ACNS 5.x.x release (except ACNS 5.4.x" can be updated

http://www.cisco.com/en/US/partner/products/sw/conntsw/ps491/products_upgrade_guides_chapter09186a00803edf35.html#wp1041284

Thanks in advance for your help!