Demodulation with MATLAB
You are asking 2 questions:


How to extract those labeled frequencies and remove the rest of the data?
How to do Demodulation?


Well, regarding your first question, a dirty solution would be by working the the DFT data.
Just zero all frequencies but those you're after and apply Inverse DFT (ifft).
Leave some margin around your wanted frequencies.

About the second question, look here:
http://en.wikipedia.org/wiki/Demodulation

Basically, in order to shift data in the frequency domain you multiply by pure harmonic signal in the time domain.
You chose the frequency of this harmonic signal s.t. (At least this is the classic case) the frequency you want will move to the DC frequency.
Then you apply Low Pass Filter to leave only the data you wanted.

This is the opposite of the Modulation process.