Before flashing, check the hardware version of the device. Updating the firmware from the wrong version of the device can spoil it!
Also, you can not update the firmware via Wi-Fi and turn off the power of the router during the firmware!
To upgrade the firmware you need:
1) Download the archive with the new firmware from the official site and unpack it.
https://www.tp-link.com/en/download/TL-WR740N.html
2) Open the router settings 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 Upgrade”, click the “Browse” button, select the previously downloaded firmware file. Click “Upgrade”. Wait for the installation to complete and until the device automatically reboots.
Is done. After updating the TP-Link TL-WR740N v5 firmware, the settings were reset to factory settings.