It is a little easier to reprogram the CHR-6d if you have the development board, since there is a jumper to pull the "boot0" pin high, and since communication between the IMU and your PC is handled (there is an RS-232 level shifter on the dev. board). There is also a 3.3V regulator on the dev. board, which is always convenient...
The little jumper on the bottom of the board (right beneath the IMU) is what allows you to reprogram the board. In the above picture, the jumper is on the left two pins - this is NORMAL mode. To activate the serial bootloader, move the jumper to the right two pins and cycle the power. This causes the bootloader to load instead of the normal firmware. When the IMU is in this mode, it is ready to be reprogrammed.
You'll need to download the Flash Loader Demonstrator program from http://www.st.com/mcu/familiesdocs-110.html. There is a lot of content on the aforementioned page, so search for "Flash Loader" to find it quickly.
The above picture shows the dialog that opens when you run the flash loader. Select the COM port that you are connected to. The baud rate doesn't really matter, since the bootloader does automatic baud rate detection. A faster baud rate just means that you'll program the device faster. Once you've selected the COM port, click "Next." If the device is connected properly and the bootloader is running, then you should see a screen like the one shown below. Go ahead an click next.
A screen like the one shown below should be displayed. This gives you a chance to select the device you will be programming. The one selected should be correct. Click next.
You should see a dialog like the one shown below. This allows you to select the .hex file that you will be downloading to the device. There are a number of other options you can toy with later, but for now just select the .hex file you want to use and click "Next" to start programming.
If you seen the screen shown below, then you have successfully reprogrammed the CHR-6d. Congratulations!
If you don't have the development kit, you can program the IMU by following the same procedure. Just make sure you provide a stable 3.3 V supply voltage, that you pipe the TTL outputs of the IMU through a level shifter, and that you pull the boot0 pin high before applying power.