How do I reset the TL-WA850RE to its factory default settings?

With the extender powered on, use a pin to press the RESET button until all the LEDs turn on momentarily, then release the button. Alternatively, you can log into the web management page, go to Settings > System Tools > Restore & Backup, and click Factory Restore. Note that you will need to reconfigure the extender to access the Internet after a reset.