Free Up Space in /boot on Ubuntu
If you notice your /boot directory on Ubuntu is getting full, these steps can help relieve some space.
1. Find the current kernel version to avoid accidental deletion
$ uname -r
2. List all installed kernel versions.
$ dpkg --list 'linux-image*' | grep ^ii
3. Uninstall all kernels whose versions are two or more before the currently installed version.
# apt-get remove ...
4. Clean up old headers
# apt-get autoremove