this is kind of good news/bad news. For access to the firmware images over https://support.fortinet.com you would need to have a valid support contract. Good news is, you can purchase a support contract based on your needs over an official Fortinet Sales representative or partner, you can find one in your vicinity on this site:
Also, good news is that you should be able to connect over SSH or directly via console cable to the device and log in to it, you should be able to configure it as you would like.
Bad news would be that the FortiGate 60D only supports firmware upgrades up to the latest of the FortiOS 6.0 branch, 6.0.14, and 6.0 branch will reach End Of Support on September 29th 2022, and no newer firmware branch versions are in plan for that model.
You can check over SSH/console with the CLI command "get sys stat" which firmware version you are on currently, and with this tool see which firmware upgrade path you would need to take: