You can download FortiClient 6.10 from the official Fortinet website:
Only use FortiClient 6.0.10 (6010) in isolated, legacy environments where upgrading the FortiGate is not an option. forticlient version 6010 download link
: Access the Fortinet Customer Service & Support portal . You will need a registered account with an active FortiCare license to see older files. You can download FortiClient 6