REPL stands for Read Evaluate Print Loop, and is the name given to the interactive MicroPython prompt that you can access on the ESP8266. Using the REPL is by far the easiest way to test out your code and run commands.
There is an USB-to-serial adapter board which be used to access the REPL on the badge via the serial port. However, a simpler option is to use the WebREPL:
WebREPL allows you to use the Python prompt over WiFi, connecting through a browser. The latest versions of Firefox and Chrome are supported.
Before connecting to WebREPL, you should set a password and enable it via a normal serial connection. Initial versions of MicroPython for ESP8266 came with WebREPL automatically enabled on the boot and with the ability to set a password via WiFi on the first connection, but as WebREPL was becoming more widely known and popular, the initial setup has switched to a wired connection for improved security:import webrepl_setup
Follow the on-screen instructions and prompts. To make any changes active, you will need to reboot your device.
You can order this USB to serial adapter board for the badge and then follow the instructions to build and flash the new firmware. The WebREPL option will turn on WiFi and put the badge into AP mode. Connect to it from your device, such as a laptop, and then connect to the badge using the WebREPL browser-based client.