Well when you say installed, do you mean like games or homebrew? Because while games and other content of that sort can be installed to the system, homebrew and the like exist solely on your SD card. If its the latter, its as simple as deleting everything on your SD card. If its the former, then you will have to do it manually from within the console itself. Either way, updating CFW is as easy as downloading the latest version of Atmosphere from the
official GitHub page. If you want to update the firmware of your emuMMC, you can use the homebrew app Daybreak which comes bundled with Atmosphere by default. Usage is simple enough that even a noob can figure it out. The tricky part is you must download the firmware files of the firmware you wish to update to seperately. Since they are considered copyrighted material, no help can be given on where to find them but rest assured they are readily available on the Internet.