It isn't, really. While there are uninstall instructions, they are not necessarily safe or reliable and they probably won't reset your console exactly to factory settings; they might also make it more difficult to re-install CFW in the future. It is very much not recommended. See https://gbatemp.net/threads/about-removing-b9s-and-the-safety-test.541718/ .is it possible to safely remove all CFW in the future if I want to reset my console to factory settings?
Of course, most people will tell you that there's really no reason you would ever want to uninstall CFW.