Downgrade 4.78 to 3.55

Hi mates,

I've got a PS3 Slim CECH-2504B under 4.78 and I would like to downgrade to 3.55 to jailbreak it.

I tried several times with YouTube tutos but I always had Corrupted File while I updated the system.

Is there someone who could post me the right files with links with the right steps.


Here's more infos about my console :

PS3 Slim CECH-2504B, Datecode 0C, 250GB, EUROPE
Firmwire : Original Sony 4.78
I've checked and my console is compatible for a downgrade to 3.40