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.

Link:
https://www.htcp.net/4351.html