I found a good tip from the matlab community to find out the built-in function in Matlab. You can edit and change it. The tip as follows:
% You can use the [which] command to find the path name to the % built-in function in question, i.e. - which fft % then point Matlab to that folder on your computer so that % the fft.m file is in your "current folder" window. Then use % the  command to edit the function, i.e. - edit fft % The code for the fft function will appear in the editor window. % Often it is useful to make small changes to the built-in code % then comment out the changes as soon as you're done so that % Matlab will continue to function as designed.