After getting the DSM 6.0 upgrade, I could not login as root using SSH.
Even if I could prior the upgrade.
1. Log into the DSM Web interface.
2. Create a new user that belongs to the Admin’s group.
3. SSH to your Synology as this new user.
4. Enter command “sudo su” and providing admin password.
5. Enter command “synouser -–setpw root ‘yourpassword'”.
As now, you should be able to SSH using the root account and apply changes as needed.