Are you rooted? Have you installed a custom recovery (ClockworkMod)? I am not an expert, but it says the file is a flashable zip. I take that to mean you enter your recovery and choose "Install zip from SD card." Follow the procedure choosing that .zip file. I'm not sure root is necessary although to rewrite any file, it probably is, but a custom recovery is necessary. Read the section on Recovery on the same page at xda that has the zip files.
CAUTION:Before doing this, run a backup of your phone in the recovery mode. It should place the backup on your ext sd card. After the backup is finished, check to make sure you have the backup and that the file size looks right (hundreds of MB's). Make a copy of that backup on your PC or use another card for flashing (place the .zip file on the root of your SD card not inside any folders). If anything should happen to the phone, you can run "restore" in recovery mode to put your phone back the way it was.