Otherwise skip the backup first, then do the following.
- Use the NUS Downloader to download the latest version of the Mii Channel (Database... -> System -> Mii Channel -> All -> Latest Version)
- Check "Create Decrypted Contents (*.app)" at the bottom
- Click on "Start NUS Download!
- Select "Tools" -> "Open NUSD Directory
- Navigate to "titles/0001000248414341/6
- Move the "00000018.app" into the root of your SD card
- Rename it to "wuphaxBackup.app
- Start Wuphax on Wii U and press "B" to restore the Mii Channel
Afterwards you can delete the wuphaxBackup.app. But first install the homebrew channel.