log in into Service&Support portal, Download>Firmware. The 3rd tab is 'Upgrade Path'.
starting at 5.2.9, upgrade to 5.4.4 - 5.6.2 - 5.6.3
Unfortunately, the list of OS patches is incomplete. The upgrade path tool in cookbook.fortinet.com is no longer available which is a pity.
FORTINET should fix that as soon as possible! What a silly idea to break a working tool.
Now, you have to step backwards through the Release Notes included in the respective download folders.
RN for 5.2.9 say it supports upgrade from 5.2.6.
RN for 5.2.6 say it supports upgrade from 5.2.4.
So, upgrade with these intermediate versions:
5.2.4 -> 5.2.6 - 5.2.9 - 5.4.4 - 5.6.2 - 5.6.3
Instead, you could also upgrade to the stable 5.4.8 after going to 5.4.4. Now you know how to determine which patch versions to upgrade to after 5.4.4...
Always back up your config before the first and after each upgrade. Do a text diff to see which parts were reconfigured. This way you can always downgrade (which will result in a config loss) and restore.
"Kernel panic: Aiee, killing interrupt handler!"