The first step would be to acquire a stock firmware version from RCA. If you can't find one on the internet, you could try calling their customer support, indicating your tablet type (RCA 7" Dual-Core), and telling them you would like a copy of the stock firmware update image.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?