FYI, u can still use the shopping channel on any firmware. U don't need to update to 4.3. All that is required to use the shopping channel is the most recent version of the shopping channel (currently v20) and the most recent version of IOS56... so if u want u can stay on 4.2 and still use the shopping channel.
This is what you should do... start ModMii... from the main menu enter "W" to launch the ModMii Wizard
Tell it your Wii is a virgin even if its not, so that u can re-hack your Wii from square 1 (U don't need to do this assuming your Wii is already properly softmodded... but that is a big assumption, and I recommend u rehack your Wii from the beginning to make sure everything is done correctly)
It will ask you some questions, answer them... ie. it will ask what firmware u r currently on, in your case, answer "4.2", then it will ask what firmware you want to be on, here u can enter "4.2" again (or if u decide u have to be on 4.3, u can enter "4.3"). When it asks you to install the shopping channel, say "Y" for yes.