Error after upgrade 60F in console

[Rodrishi]

good morning everyone, i have a fortinet 60F. unfortunately, a junior member did the upgrade from 6.4 to 7.15. when i try to log in console to fix it, i get this:

▒▒▒▒▒▒▒▒ђ▒▒▒▒ѥ▒▒▒ɩ▒͕▒ѥ▒▒▒ɩ▒▒▒ѥ▒ѕ▒▒▒▒Ҫ▒▒▒▒▒▒▒▒▒▒▒ɂ▒▒▒▒▒▒▒▒▒ɥ▒▒յ▒▒▒ԕ▒▒ʥ▒▒▒▒▒▒х▒(▒ѥ▒▒▒饹▒▒▒ѕ٥▒▒▒▒a▒ѥ▒▒▒饹▒s▒L▒᱕▒͕▒▒▒▒▒ɢ▒▒▒▒ɂɕ▒͹▒墽▒▒▒▒▒˙▒▒▒Ʌѥ▒▒▒▒▒▒▒▒▒▒▒▒ѥ▒▒▒ɩ▒ʥѥ▒▒▒饹▒▒ɕ݅▒▒▒ɩ▒failed verification on /data/datafs.tar.gz
fos_ima: System Integrity check failed....
CPU5: stopping
CPU6: stopping
CPU1: stopping
CPU3: stopping
CPU7: stopping
CPU0: stopping
CPU2: stopping

sadly, it keeps repeating this message and it doesnt let me type commands. any advice? i tried changing the speed and flow control of my console cable with no better result than this one. my current best settings are:

speed 9600 - 8n1, no flow controls

[ede_pfau]

These symptoms may indicate that the update did not follow the Recommended Update Path.

Have a look at this KB article for background info and remedy:

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Boot-failure-after-upgrading-to-v7-0-15/ta...

BG info to the Update Path requirement: a firmware update often is not only new code. It may contain macros which modify the current configuration to match changed syntax, for instance. In order to preserve the configuration, each intermediate fimware version in the Rec. Update Path needs to be applied.

Also, this is the reason why downgrading may profoundly damage the configuration as there are no "reverse macros" included.

Maybe this info will help you and your colleagues to understand why the Rec. Update Path should be respected. You can reach it's web site via the Firmware section of the Service Support site (behind login portal) or just https://docs.fortinet.com/product/fortigate/7.2 (where "7.2" is arbitrary).