Kernel is basically what is actually different from firmware to firmware, it's the programming 'n' such that's actually different from version to version. Messing with the kernel is completely unnecessary as long as you have everything in the right folders- 1.5 and 1.0 homebrew in the GAME150 folder, PSX games in the GAME folder, 3.40 or 3.51 homebrew in the GAME340 or GAME351 folder, and ISO files in the ISO folder. As long as you have everything in the right folder everything will work perfectly- if you have an ISO that takes 2.0 FW then just put it in the ISO folder and it will work perfectly. If you still want to mess with it (for curiosity or otherwise) it's in the recovery menu. I dunno exactly where, hold on, I'll check... Configuration>Game Folder Homebrew, that's where you can set it, although I wouldn't personally recommend doing anything if you just want stuff to work right.