Hi,
You can download 90-day trial version at http://www.cisco.com/public/sw-center/sw-ciscosecure.shtml,
But now you need to have CCO acount (and service contract) as stated on the page.
The ACS 2.6 was already EOS/EOL since 2001.
All newer ACS 3.0 or higher runs only on Windows 2000 Server. If you are upgrading from a previous version of Cisco Secure ACS that is running on Windows NT 4.0, you cannot upgrade the operating system to Windows 2000 Server. This is because the setup program for previous versions of Cisco Secure ACS detected which Windows operating system ran on the server and customized Cisco Secure ACS for that operating system. As a result, upgrading the operating system to Windows 2000 Server without taking the necessary steps causes Cisco Secure ACS to fail.
Follow the instruction to upgrade/migrate to ACS 3.1 if your curren ACS 2.6 runs on WinNT.
http://www.cisco.com/en/US/products/sw/secursw/ps2086/products_installation_guide_chapter09186a00800e6eae.html#2345
Depending on your requirement (i.e database migration), getting a newer version instead of upgrading is probably better.
Rgds,
AK
