Change / tweak existing wordpress translations

by

It is better to use a separate file to prevent an overwriting during an upgrade.

Filter load_textdomain_mofile and replace the old file with your own:

add_filter( 'load_textdomain_mofile', function( $mofile, $domain )
{
    if ( 'name_of_the_textdomain' === $domain )
        return 'path_to_your_file.mo';

    return $mofile;
});