0x67 UBUNTU - REMOVE OLD KERNELS

If your system is operating without error, you should be able to remove old kernels with a simple autoremove command in shell:

sudo apt-get autoremove --purge

Alternatively, and to be able to remove even manually installed kernels, you could use:

sudo purge-old-kernels

Source: https://help.ubuntu.com/community/Lubuntu/Documentation/RemoveOldKernels