Depending on your release (and maybe even OS), you might be able to edit the internal Matlab function, but you shouldn't. It is best to modify your code to correct for the difference between what you need and what you get. (the mod function can help with the wrap-around)
Editing internal functions is a bad idea, because it removes any form of portability. If you choose to re-install Matlab, or install a newer release, you will have to re-apply the change, which might not be possible. You might also break other function (both internal function and function written by others (teachers/FEX submissions)).
