Troubleshooting Tip: FortiGate Supported Legacy Upgrade Paths

More details is needed to understand this chart.
I have an old FortiGate-50E on version v5.4.1,build1064 and I want it towards v6.2.17.
The chart on this page shows at most v6.0.1.

I dont understand the legacy upgrade path on this page and I cant see how to get it fromv5.4.1 to v6.2.17
Can anyone help me?