A new PC tool from Datel, My Mii Manager, allows you edit your Mii's, usually done within the Mii Channel, on your PC.
You can pass you Miis from your Wii to your PC by first uploading them to the memory on your Wii Remote, then connecting your Remote to your PC via a bundled Bluetooth receiver.
Edited Miis can then be saved onto your PC ready for re-uploading to your Wii Remote, or you can send them to friends via the internet.