In Settings -> About tablet -> System updates, you may upgrade your tablet to this version automatically online or download the firmware 4.0.5 and use the recovery mode to install this firmware by "Update Firmware". However your previous root may not work. Thanks to surdu_petru from xda-developers.com who developed a tool for new rooting.
After the tablet is SDE enabled, the procedures are:
- Download ICS_405.zip and drive_405.zip and unzip;
- Connect the tablet to your host machine and start adb by "adb devices";
- Copy archos.ext4.update into Internal Storage by "adb push archos.ext4.update /data/media";
- Power off your device;
- Boot into recovery by holding the volume rocker down and pressing the power button simultaneously you should now be in Recovery mode and select "Developer Edition Menu";
- Remove Android Kernel" (if the option is present, if not, skip);
- Select Flash Kernel and Initramfs;
- Copy zImage and initramfs.cpio.lzo to the drive that appears;
- Reboot your device. Your device should now be rooted.
If the tablet is not booting for some reason, you still can use the recovery mode to reinstall the firmware 4.0.5 by "Update Firmware" again.