Never update firmware over Wi-Fi. Use an Ethernet cable to prevent the connection from dropping during the update.
Ensure the firmware version matches your hardware revision (usually found on a sticker at the bottom of the router). Flashing the wrong version can "brick" the device. Download KASDA KW5515 Firmware rar
The safest bet is the Kasda Support Page. Look for the "Download" or "Firmware" section and search for . Never update firmware over Wi-Fi