Ignore random “driver download” websites—they are full of malware. Go straight to the source.
: If the router acts strangely after an update, perform a factory reset and restore your settings.
To update the firmware of your TL-WR840N router:
The TL-WR840N is unique because it went through several hardware iterations (v2, v3, v4, v5, v6). The "best" firmware is strictly tied to your specific hardware version. You cannot install a v5 firmware on a v6 router; doing so will "brick" the device.
The Ultimate Guide to TP-Link TL-WR840N Firmware Updates: Best Practices for Performance & Security