downgrade to custom previous android os version [gt-i9190]
This definitely a fake phone. The problem is where do you want to get that OS? There is no support offered by those chinese firms which are faking phones.
My advice: Either get a proper, not faked phone or just stick to what you have. You may be able to uninstall some bloatware and you are good to go with that phone. It will not be as powerful as the original but it may help out.