You will first need the mBed serial port driver available on your machine. In Windows 10, Windows Update will fetch the driver. 

For earlier versions,  or if you have used mBed before and upgraded, you may need to re-install the driver. 

Once you have installed this driver, the micro:bit will also appear in device-manager as a COM port, and you can use any terminal emulator program to send and receive data to and from the micro:bit over the USB serial lead, using the 'serial' language capabilities of many of our standard coding languages.

You should also set the baud rate to 115200.

When installing the Windows mbed driver, you should make sure that the micro:bit is plugged into the USB first, before trying to install the driver - some users have experienced 'unknown install errors' of this driver, and it seems to be related to not having the micro:bit actually plugged in at the time you do the install. You will also need full admin rights on your PC to install the driver. 

Keywords for search: USB, serial port, Windows, REPL, Mu