You can also get a new led pre-mounted on a board and skip the reflow. Just solder the wires.
Reflowing is a soldering method, as opposed to many other. High power LEDs are typically mounted on MCPCBs by reflowing. A simple explanation would be that the solder is placed first, the component set in place then, and only after that is heat applied. There are several procedures you could follow to reflow @home, some of them state of the art, others more or less ghetto style.
You don't really need to reflow anything to replace a single emitter. Actually I would even advise against that in this case. It's easier and not much more expensive to replace the LED-MCPCB assembly with a new one.