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.
Sites like ADSLZone or OpenWrt (if you are looking for custom firmware) sometimes host mirror links, though you should exercise caution with .rar files from unofficial sources. Important Precautions Download KASDA KW5515 Firmware rar
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