overwrite function (meta_form)

I want to make a theme where I overwrite the admin “Custom Fields” area with some HTML/PHP tweaks.

So on the functions.php page I define a new function, and I want to overwrite the function “meta_form” in includes/template.php

What am I doing wrong?

function new_meta_form() {
echo "new content";
}

add_filter('meta_form', 'new_meta_form', 1, 2);