How do I solder a LED in place of an earphone?
I think you're asking the wrong question. What you evidently want to do (as suggested by your previous, now closed, question) is light up an LED over bluetooth.

Connecting an LED to the audio output of a bluetooth headset is very unlikely to work in the way you'd like it to. To adequately light an LED, you need perhaps 20mA of at least 3V DC. The drive for a small earphone like speaker found in a bluetooth headset is 1. AC (the direction of the current varies over time), 2. very low voltage (this SO question suggests about 0.3V RMS) and 3. is unlikely to be able to provide the current provided. In summary, it is exceedingly unlikely to work in any meaningful way.

So what's the alternative? First, you need to define the problem more succinctly. The other end of the bluetooth connection will be a computer of some sort. What is it? The requirements will be quite different if you're looking to control something from a mobile phone, a Windows PC, a microcontroller or any other computer. A quick example I found by googling, this Bluetooth GPIO module would be very easy to control from a PC (they even provide software samples). Controlling it from a phone would be harder - they don't provide software, and so you'd have to figure out exactly what their protocol is. There's also fairly low cost Bluetooth to serial devices which you could pair with a microcontroller (eg an Arduino) and then define some commands to control the microcontroller pins.

The hard part of your question is really software related. You need to define your controlling platform before you can progress any further.