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I installed 18.04 after 14.04. The default grub entry was 18.04 and 14.04 was displayed below. After a 14.04 update, things changed.
Instead of booting into 18.04 advanced options and choose the first kernel before last and then remove the last linux image and headers, reboot again in ubuntu 18.04 and make an update to the last kernel.... I tried to delete the unused linux image and headers hoping that will do the job. It didn't.
I tried 'sudo update-grub' in ubuntu 18.04 and then reboot, but no change. (I use 18.04 ubuntu-mate and 14.04 ubuntu).
Is there a way to put 18.04 as default, besides waiting for a new kernel update in 18.04?