If you, like I also did, locked yourself out like in this thread:

https://forum.synology.com/enu/viewtopic.php?f=90&p=424906

I got this error:

sudo: /etc/sudoers is world writable
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

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I do not support and can not be held responsible for any damage this would do to your device. I only state that it worked for me.

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SOLUTION

Now I explain what worked for me.

Install Config FIle Editor package in your DSM-package center. It can be found here.

Now you can use Config File Editor to edit your versionnumber and do a downgrade.

First add this line to the config file editor

/etc.defaults/VERSION,version

Close down the Config File Editor and open it again. Choose version in the drop down.

Change buildnumber to eg 7100.

Now you can do a manual downgrade to beta 1 – this gives you your root access back (find the update icon in control panel and do a manual installation af the DSM 6.0 beta 1)

Find your DSM 6.0 Beta 1 here:

http://usdl.synology.com/download/DSM/beta/6.0/7135/

It bricked my Synology. Now I was not able to access my DSM.

But I still/now could login with my root login and pass through SSH.

So login to your NAS with your normal root login and password. (I use PuTTY as explained here – use ssh and type the local ip to your NAS).

Change sudoers chmod to 0440 in the etc-folder

cd etc

chmod 0440 sudoers

Thereafter you are able to upgrade from command line with this command, that I found here.

synoupgrade 
Copyright (c) 2003-2012 Synology Inc. All rights reserved.

   --auto
   --check
   --download
   --start
   --parse
   --patch ABSOULATE_UPGRADE_FILE_PATH

I used synoupgrade –auto (that worked, 10-15 min. later it was updated and accessible again)