The only directory that you can clean up without any problems is the /var/opt/novell/Sentinel/log directory.
It is not much but it might be just enough.
Once sentinel has started go to storage -> Data synchronization and disable all RDD's. Better still first open all RDD's and put the retention date to 2 or 3 days or something and leave it for a day or so.
Delete everything in the security intelligence dashboard, this will take a few days to be effective.
Put the data retention on all the policies on the at least field on 2 days or something like that.
After you have gained some disk space again, upgrade sentinel.

After all this, have a critical look at what you really need.
On a system with not much disk space you probably don't want the security intelligence because it fills up the mongodb.
The RDD's fill up the postgresql db so be carefull on which one to use and what the retention date is set to.
Finally you can make additional data retention policies to delete unimportant data more quickly.
One thing I normally do is putting the raw data retention policy to at least 1 day and at most 3 days to get rid of the raw data as soon as possible but yet being able to back it up to tape. If you really don't need the raw data (sentinel only stores it as a reference but doesn't do anything with it) you can also switch of the collection of it all together. This will also gain you a little bit of performance, but I experienced that I have to remove the data by hand now because the data retention policy is not deleting the data any more.

One other thing you can do is splitting the partition you have your data on in two and use 1 partition as a network storage. The data on the network storage is compressed more heavily then on your local storage. Keep in mind that at first it will take up more space because events are stored mostly uncompressed on the local storage AND compressed on the network storage.

Hope this helps.