Upgrade Path for Firmware on FortiSwitch

I would like some input on the best path to take for the upgrade of Firmware on my FortiSwitches. I’ve got 124D’s that are on Firmware as old as 3.5, 124E’s and 124E’s that are on 3.6. What is the best upgrade path to take to get them on 6.2, considering that they are not currently in managed mode? 

To clarify, I’m looking to find out if I can upgrade from 3.5 or 3.6 to 6.2? I know the support website will tell me the path for the FortiGate, but I can’t seem to find the same info for the Switches. Thanks.