you can try to look in /var/log, probably several old log files can be deleted (those ending. with .0 .1 .2 ... and ending with .gz),
because of a know bug in logrotate package probably many log files are grown ... you can zero out these files using a command
additionally, file in /tmp can be safely deleted.
look in /root/tmp too, and delete temporary files.
Of course, the real solution is to upgrade to a larger disk.