Ubuntu - Kernel - Delete a newer kernel while booted with the older kernel

Get a list of the kernels that are installed

dpkg -l | grep linux

sudo apt purge linux-...<name as shown by dpkg -l>

NOTE: The headers do not need to be removed, the linux-image-… are the actual binaries that need removal.

The apt should automatically re-run the necessary commands to update grub and the startup RAM disk. However you can run these manually if needed:

sudo update-initramfs -d -k $(uname -r)
sudo update-initramfs -u
sudo update-grub