The latest version is 12.9 but a direct upgrade from 12.5 to 12.9 is not supported. You need to upgrade to 12.5 Sp1 or 12.8 first before upgrading to 12.9. The 12.9 DVD image can be downloaded from Support Online. The 12.5 Sp1 and 12.8 images are no longer available on Support Online. Please create a regular support case and asked for one of these to be made available to you.
The upgrade process is described in great detail in the Client Automation Implementation Guide chapter 6. You can find this guide in the Doc directory on each dvd image.
regards,
Leo