Might as well get 4.1 while you're at it
It only has good changes, unlike 4.2
I updated to 4.1 today with a method I came up with on my own:
1. Install all missing IOSes with DOP-IOS and patch the hash check (but don't update ones you already have), so you're sure you have all IOSes required for any current game or system menu.
2. Install the system menu WAD of your choice.
However, I have to warn you before you try anything - WAD Manager is NOT safe. I found this out when it failed in the middle of the installation and bricked my system menu. Luckily I had BootMii to save me, or I would have never messed around with updating the system menu unofficially in the first place.
A friend followed the same method yesterday with the same system menu WAD, and it worked fine for him. It also worked for me on the second try, so it was just a random hiccup from WAD Manager. EDIT: I made the mistake of rebooting the Wii when it failed installing the system menu wad though, so if you don't make that same mistake you would be able to just press install again and it would work
With that in mind, feel free to use the steps above as they are quite simple and probably not any less safe than Wanin's firmware updaters, although I haven't tested them myself (I'd rather not).
Just make sure that if you use any unofficial updating method, have BootMii boot2 beforehand (and a SD card that works with it)
A backup is not essential as you can just start WAD Manager from cBoot2 and reinstall the wads you need to if anything bricks, however it does make unbricking a lot easier
EDIT: Phew, that was a lot of text. There were no other posts in this topic when I started