There is no reason to update to SM 4.3. SM 4.1 was the last SM version to offer something that wasn't just homebrew-prevention. All that matters is that you have the latest IOS/cIOS files installed, and for that you can use ModMii.
If you some reason you still want to update to SM 4.3, even though it is not recommended, make sure you have the latest version of the Homebrew Channel, make sure your Wii is connected to the internet, and update using a normal online update. If that does not update you to SM 4.3, then you might not be connected to the internet or something like Priiloader might be blocking the updates. If you do successfully update to SM 4.3, you might have to use ModMii to reinstall some cIOS files, bootmii, etc. that might be overwritten during an official update.
You can also use ModMii to update to SM 4.3, but as I already said, the System Menu really doesn't do very much, and there's no reason to update to anything higher than SM 4.1.