Do you need help a specific error during the update process?

The "latest" firmware for a FortiGate 200D is the highest patch level of FortiOS 6.0. While this keeps the device functional and safe from known vulnerabilities, it represents the end of the road for this specific hardware. Ensure you are at least on the latest 6.0.x patch to keep your network defended until your next hardware refresh cycle. To help you finalize your update or hardware plan:

Tell me your current and I can provide the exact steps.