First, check the hardware version of the device, it is indicated on the label under the router. Upgrading the firmware from the wrong version of the device can spoil it! You can not update the firmware via Wi-Fi and turn off the power of the router during the firmware!
I will describe the process of updating the firmware:
1) Download the archive with the new firmware from the official site and unpack it.
https://www.tp-link.com/ru/download/TL-WR340G.html
2) Open the settings of the router by typing the address http://192.168.0.1 in the browser and enter the username – admin, the password – admin.
3) In the menu, select “System Tools” – “Firmware”, click the “Browse” button, select the previously downloaded firmware. Click “Upgrade”. Wait for the installation of the firmware and the device will automatically reboot.
After updating the firmware, the settings in the router were reset to standard ones.
Done.