To summarize, to update to 6.0, you'll need to incrementally go to 5.2 first. Also, when updating to 5.2, you'll need to migrate your license to a new one issued by Cisco. Instructions for retrieving the new license are provided in the release notes (link below).

Additionally, prior to running the updater to 5.2, you'll need to back out any HotFix or special extensions added to the system. The updater will let you know whether this is needed when you run it on the system.

Here's the link to the 5.2 release notes:

http://www.cisco.com/en/US/docs/app_ntwk_services/data_center_app_services/ace_xml_gateway/v52/release/note/xmlrn52.html#wp39194