go to support.fortinet.com, Log in and go to downloads -> Fiirmware Images. There, you can get the upgrade path for any Fortigate.
If budget is tight, consider virtualizing FortiGate-VM (which runs the latest firmware) on inexpensive hardware. fortigate 200d latest firmware
The FortiGate 200D was once the gold standard for the mid-enterprise and large branch office market. Known for its "D-series" robust hardware and Fortinet’s proprietary SPU (Security Processing Unit) technology, it offered throughput that was impressive for its time. However, running this unit in 2024 presents significant challenges regarding security compliance and feature support. go to support
If you have a 200D running an ancient version like v5.2 or v5.4, you cannot jump directly to v6.0.15. FortiOS requires an incremental upgrade path. go to support.fortinet.com