If you have fdesign:

Fs = 44100; % Sampling Frequency

N = 2; % Order

Fc = 3500; % Cutoff Frequency

h = fdesign.lowpass('N,F3dB', N, Fc, Fs);

Hd = design(h, 'butter');

Then you can filter(Hd, your_signal).

Otherwise you can calculate a biquad low pass filter like this:

w0 = 2*pi*fc/Fs;

alpha = sin(w0)/(2*Q);

b0 = (1 - cos(w0))/2;

b1 = 1 - cos(w0);

b2 = (1 - cos(w0))/2;

a0 = 1 + alpha;

a1 = -2*cos(w0);

a2 = 1 - alpha;

b = [b0/a0 b1/a0 b2/a0];

a = [1 a1/a0 a2/a0];

You then filter with filter(b, a, your_signal). Q would be 1/sqrt(2) for butterworth.