Programs enabling conversion of the file:
If your objective is to convert the file from FB2 to the file RB, you can use any of the applications listed. FB2 to RB conversion process in every program intended for this purpose (so-called converters) is very intuitive and everyone is able to manage converting a file.
However, bear in mind that the list may also include programs to support and work with the indicated extensions FB2 and RB - and not only converters. In such a case, you need to open the source file FB2 and from the options available in the program, choose ”Save as... RB" or "Export to RB".