I have only 6 free Mb on "boot volume".
How can I find for not-used files to remove?
Or is it possible to add memory to this unit?
Your boot partition may contain older images of Linux kernals - they can be removed, and there is a procedure to do this outlined in What is the safest way to clean up /boot partition?
Update 2014-06-28
It looks as though you will need to increase the size of your /boot partition. This is not terribly fast, and of course if something goes wrong you will be glad you made a backup!
It may actually be / faster / easier / better all around / to reinstall Ubuntu.
You cannot resize partitions while you are using them, and Ubuntu is using them right now. So, you will need to boot from a live USB, and choose 'try Ubuntu'. When that is running, you open a terminal window and enter
sudo apt-get gparted
This will load the partition manager onto the USB. You can then run the partition manager to resize the partitions of you hard disk.
You want to make your /boot partition (/dev/sda1) larger - to do this you need to have some space available right next to it. To do that, you will probably need to shrink another partition, and possibly move partitions on the disk as well. It's the resize and move which take a long, long time.
I'm attaching two links to relevant posts made in the past - good luck!
http://www.howtogeek.com/114503/how-to-resize-your-ubuntu-partitions/
df -h /boot
and du -h /boot
Jun 27, 2014 at 22:06
One option that hasn't been mentioned, is to keep /boot on your / partition. If you regularly update your kernel and you keep your /boot on a separate 100-200MB partition, you will have a lot of clean up to do. Keeping it on a sufficiently sized / partition, you won't run into troubles so easily.
On most (desktop) systems these days, there's no real reason why you should keep a separate /boot. Of course, some people will disagree, but it's more of a legacy problem that's causing the habit of creating a separate ext2 partition for /boot.