I forgot the root password of my NAS Which is running Debian.
After some Googling I found a easy way to reset it.
Press "E" on your grub menu and add "init=/bin/sh“ in the end of the "Linux" line. and change "ro" to "rw".
Press Ctrl + X to login and change your root password
passwd root
Turn off and turn on again(Power button).
Done.