I can no longer update ubuntu 16.04 or upgrade to 18.04 as I need to delete some space in /boot. I normally delete the old kernels but the oldest is now 18.104.22.168 which appears to be the running kernel in uname. My latest downloaded kernel is 22.214.171.124 according to dpkg -l. I did fairly recently rolls back to an older kernel as the system didn't fully boot which didn't solve the problem (I created a new user instead). So I may have blindly followed some instruction and caused the current problem. 'Update-grub' and reboot did not solve the problem. As you can see I'm not an experienced user. How do I run kernel 126.96.36.199.
Other information: when I do remove old kernels with apt-get purge linux-image... ,I do get some error messages: Error! Could not locate dkms.conf file. File: does not exist. and The link /vmlinuz.old is a damaged link Removing symbolic link vmlinuz.old you may need to re-run your boot loader[grub] The link /initrd.img.old is a damaged link Removing symbolic link initrd.img.old you may need to re-run your boot loader[grub]