I have a N3DS 10.4 and I want to Downgrade the Firmware to 10.2 (or lower?) so I can use NASA to inject Cubic Ninja through browserhax
You won't be able to downgrade form 10.4 without a hardmod. Downgrading via software is limited to 10.3, so unless you can find a hardmodder or do it yourself, (very unrecommended unless you have access to a soldering kit and prior experience.) you're SOL.
You could ask hundshamer to hardmod for you, but since you're allegedly from argentina, shipping will take a very long time and be quite costly.
I want to update my N3DS to 10.5 with Ninjax on
Ninjhax is not a legit cia. NASA will not process it. You'd have to downgrade to 9.2, which means you'd have to downgrade to 10.2-3, which means you need a hardmod at this point. If you've got a hardmod, it's better to just install emunand.
Very unlikely. Maybe possible if you have enough money, soldering experience, or patience to make it happen.