Loading MicroPython on the BBC micro:bit is very easy.  When you plug the board into your computer it will appear as a USB storage drive.  If you drag a firmware .hex file to the drive it will be programmed to the board.  You can use either the online BBC micro:bit code editor or the desktop mu MicroPython code editor to enter MicroPython code and push it to the board.

Note on Windows if you'd like to access the board's REPL over a serial connection you'll need to follow the Windows CDC serial driver install steps below.

With the online editor you'll download a .hex file and drag it to the microbit's drive.

With the desktop mu editor just press the flash button with the board connected and it should automatically upload the MicroPython code & firmware to the board.

Jump to the Serial REPL page in this guide to learn how to access MicroPython's 'command prompt' on the board.

Windows CDC Serial Driver Install

To access the BBC micro:bit's serial port follow the instructions to install the mbed serial port driver.  Once installed the BBC micro:bit should show up as a COM port in Device Manager when connected.