You'll have to find some place that has the image but such things are usually so cheap you can have fun with it and try it without big heartaches as it crashes or never comes back.
google this word and you'll find forums and more about such -> CyanogenMod
http://en.wikipedia.org/wiki/CyanogenMod is the wiki about it.
There is no support, I only suggest this once you accept all the risks and well, all the risks are yours as you discover there is no support.
I just bought a generic , out of Hongkong tablet and tonight I was researching 4.2. The tablet I bought has 4.0