If it is anything beyond 3.55 for the original firmware, you are out of luck. You will need a hardware flasher. A hardware flasher isn't for the feint of heart, either, and I wouldn't recommend anyone with light hardware skills to try it. You have a high probability of turning that expensive PS3 into an expensive addition to your local landfill.
So, in short, you're are out of luck to do it yourself. You can likely find trustworthy people you can ship it to, have them downgrade it with their flasher, and then ship it back to you, for a small fee.
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I hope this isn't bad form here, but there is a good article on Tortuga Cove
covering the same thing. I followed the guide here and the one on Tortuga Cove's site. The result was a clean upgrade from 3.55 Kmeaw to 3.55Rogero+4.30.
For what it is worth, I did not install the 3.55 original firmware in my upgrade process.