From the command prompt. You have to load the image file.
Download the fastboot utility linked in post #1 for your OS.
If you are using Windows (boo) download and install the HTC Android USB Driver for Microsoft PC from the bottom of this page.
Run the command as appropriate for your OS;
fastboot flash recovery /path/to/<recovery>.img (windows)
./fastboot-mac recovery /path/to/<recovery>.img (mac x86)
./fastboot recovery /path/to/<recovery>.img (linux)