Assuming that you have an un-modded JAP console you should be able to do this via the update function in the Wii settings. If you have region-modded a PAL/NTSC-U console then you will have to use something like Zektors 4.2 Safe Updater, or look for his offline guide and do it manually.
All this is based on assumption as you haven't given much info and that you are looking to get to the latest 4.2J firmware.
EDIT: I noticed that you are in the UK, Im not sure if the official update checks your IP for your region as well as the Wii's region