Place the SD card into your Kobo. Hold the dpad button down and press the on/off button. Hold the combination down until 5 squares appear on the boot up progress screen, then release both buttons.
The Kobo should go into firmware update mode. Just let the update do it's thing which will take about 15 minutes more or less. Wait until your screen reads "Firmware Update Process Complete" and then take the SD card out.
Power your Kobo up and change date/time
You will need to upload all your ebooks again.