You should always leave the current kernel and one previous kernel in place on your system. The previous kernel is needed if an update to the current kernel leaves your system unable to boot. You will be able to boot the previous kernel using the advanced options of the grub boot menu.
Show current kernel in use:
$ uname -r
Get list of installed kernels:
$ dpkg --list | grep linux-image
Remove old kernels one by one:
$ sudo apt-get --purge remove linux-image-xxx
Update grub:
$ sudo update-grub2
Reboot system:
$ sudo systemctl reboot