You can use your nand backup if u have bootmii as boot2. There is a 4.2 downgrader available but I believe its risky. Why do u ask? If u have preloader installed u can block online and disc updates so no fear of update.
If I accidentally update my system to 4.2 and then mess up everything, can I use my NAND backup or is there another way to return it to 4.1e and to turn everything back to normal?
Do be aware too that if you have bootmii/boot2 on 4.1, then an update will remove it and it will require re-installation.
Best avoid the update. 4.2 provides no benefit over 4.1, just minor drawbacks.
Not really doing much Wii stuff these days.
If you installed 4.2 and you don`t want to downgrade then you will need to reinstall any IOS over 200 and, i think, priiloader aswell. Downside is loading backup discs and out of region games may no longer work.
Downgrading is risky as the 4.1 system IOS has been stubbed.
Pob3008`s suggestion of a Nand backup is the safest in my book.