If its bricked, then no need to bother.
Otherwise, one way to do it (friend did it recently):
* Remove any custom/pirated channels using their wads. DO NOT REMOVE IOS/SYSTEM MENUS!
* If you use cIOSCORP: use the cIOSCORP uninstaller package.
* If you use preloader: Remove by reinstalling the system menu (DO NOT UNINSTALL THE MENU!)
* Use HackMii installer to remove Bootmii, DVDX, HBC.
* Update the wii via official wifi update
* Optionally format wii (I do not recommend this!)
* Remove the SD card and send it in. Use a softmod guide like xzxero's when you get it back from Big N.