MP4 Header Information In Its Binary

[sacko]

I have a small MP4 file of duration 10 seconds, less than 1MB. I am able to open the file in HexEdit to view its binary source. But I am not able to figure out where does its header information is in this binary file.
What I basically want to do is change the duration of this MP4 to say like 20 seconds. I know its a read only attribute and thats the reason why I am trying to edit its binary data to update the duration.
Any help would be greatly appreciated. Thanks..!!