ALL of the cores are just software that is uploaded to the board. So, updates are just a matter of copying the new core on to the SD card. You select the core you want to run using a built-in menu system.
The board has an open source library for its video, audio, etc. so anyone can port any existing FPGA project to the board. Several people have already ported their projects over to the board, giving us Atari 800 emulation among others.