If the serial number of Zarenbourg starts with 15 or lower and the last digits are 269 or lower, then you have to do an extensive update in several steps. You find the exact instructions as a .pdf file in the download section (available via the Home link in the upper right corner). Otherwise, proceed the following:
Download the latest firmware from the download section. Unzip and load the files onto an SD card. Make sure the SD is formatted in FAT32, otherwise Zarenbourg won’t read from the card. Insert it into Zarenbourg and power on the device.
Zarenbourgs LEDs will now blink rapidly for about a minute. DO NOT TOUCH THE INSTRUMENT during the update procedure: do not press any key or button and don’t move any of the knobs! This may render Zarenbourg inusable.
When the LEDs haven’t moved for 20 seconds, switch off Zarenbourg, but do not use any of the controls or keys yet. Remove the SD and turn on Zarenbourg again.
The update is now finished, you may continue playing your instrument.