FM demodulation technique other than fmdemod?

Fs = 5000; % Sampling rate of signal
Fc = 500; % Carrier frequency
t = [0:Fs-1]'/Fs; % Sampling times
m = cos(2*pi*20*t);
kf = 10;
theta = 2*pi*Fc*t + 2*pi*kf*cumsum(m);
FM = cos(theta);
subplot(2,1,1);
plot(t,FM);

i modulated the signal, but i need some help with demodulating it without using fmmod or fmdemod, any suggestions?