FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article explains how download the Firmware of FortiGate manually into Fortinet's website and how to upload it to FortiGate.

On this article, the FortiGate is on 6.0.5 build 0268, and the aim is to download Firmware 6.0.6 build 0272 and upload it to the unit.

Go to then login to your account.
Go to Download –> Firmware Images

Choose 'FortiGate' under 'Select Product', click on the 'Download' Tab and Choose 6.00 Folder

Choose the 6.0, since in this example the goal is to download the 6.0.6 build 0272 Firmware Version.

On Folder 6.0, choose the 6.0.6 build 0272 Folder.
Then look for the accurate Fortigate model (Example in this case: FGT100D).
Download the .out File by selecting the HTTPS.

Once downloaded, Access the FortiGate GUI.
Go to Systems –> Firmware.
Then click Browse to upload the downloaded Firmware.

Choose the Firmware that was downloaded.
Then, click on the option 'Backup config and upgrade'.