Here's the code I used to try and write the file. Its still in the game but doesn't work for some reason:
file = file_text_open_read(working_directory + "\highscore.txt");
file_num = file_text_read_real(file)
show_debug_message(file);
file_text_close(file);
if (score > file_num) {
file = file_text_open_write(working_directory + "\highscore.txt");
file_text_write_real(file, score);
show_debug_message(file);
file_text_close(file);
}