How to write .his file

Hi,

I manage to read .his file with following commands

function [buf]= ReadHis(filename)

fid= fopen(filename, 'r'); % open file for reading ('r')

if (fid);

    pn= fseek(fid, 100, 'bof');   % seek to the 101th byte,
    buf= fread(fid,[512 512], 'uint16');  % read 512x512 16-bit unsigned integers into variable 'buf'
end; 
fclose(fid);

After reading the .his data, I did required modifications. Now I have 512x512 data. How can I write it back into .his format? Any suggestions and guidance will be appreciable.

Thanks, Rami