I assume by upsampling you mean inserting zeros to increase the sampling rate and applying the interpolation filter.
Have you tried resample.m in the Signal Processing Toolbox?
% Original sampling rate is 1000 Hz
t = 0:0.001:1-0.001;
x = cos(2*pi*100*t);
% resample at twice the original rate
y = resample(x,4,2);
If this does not help you, it would be helpful for you to post some code to show where you are having trouble.
