If you already have the HBC, launch the HackMii Installer via the HBC to reinstall and\or update the HBC or Bootmii.
If your Wii has Bootmii as boot2 installed, you can insert the SD Card with your ModMii files into your Wii and turn it on, and Bootmii will load. From the Bootmii menu, press the power button on the Wii console 2 times to get to the third option (the SD menu), then press the reset button to select it (you can also use a Gamecube controller). Then press the power button repeatedly until bootmini.elf is selected, and then press the reset button to select it. This will load the HackMii Installer allowing you to reinstall and\or update the HBC, and Bootmii.
If you already have a Forwarder Channel from a previous softmod, just save the boot.elf/dol corresponding to the app you want to load (e.g. SD:\apps\hackmii_installer\boot.elf or SD:\apps\MMM\boot.dol) to the location used by your specific Forwarder Channel (e.g. SD:\apps\usbloader_gx\boot.dol). Then start the channel and the HackMii Installer will load allowing you to reinstall the HBC and Bootmii. Note that if your forwarder channel only loads dol's and not elf's, launch MMM's boot.dol, then use its "App Manager" to load the HackMii Installer.