This guide will provide the strategy to legally obtain the final, stable firmware (FortiOS 6.0.15 or 6.2.10) for your 100D, ensure integrity, and perform the upgrade safely.
You cannot simply upload 6.2.10 over 5.2.0 . Here is the mandatory path: Fortigate 100d Firmware Download BEST
If you want, I can also list the to look out for with the 100D. This guide will provide the strategy to legally
Installing firmware — GUI method (standard) Fortigate 100d Firmware Download BEST