Auto update on Visual composer plugin works only if you have your own private license key. If you are using our default developers license, than automatic update won't work. 

The easiest way to update your Visual composer to new version is to simply first deactivate it, than delete it. After that you should see a notification in backend:

After clicking "Begin installing plugin", the latest version of Visual composer from our servers will be downloaded. If for some reason this notice doesn't appear,  you will find it in "Install plugins" under Apperance in your backend.

Notice: Visual composer is a plugin from We at can't influence how it behaves and we can't be accountable for any feature or code changes in new versions. Make sure you always make backup before updating. Backups are the only reliable way to prevent any data loss.