Default Kernel

List all available kernel

sudo awk -F\' '/menuentry / {print $2}' /boot/grub2/grub.cfg

:!: Check if all the kernels listed in order and properly set the version (latest on top) if not generate config again.

Re-Generate config

grub2-mkconfig -o /boot/grub2/grub.cfg

:!: Redo the previous step and confirm all are listed as expected.

Check default kernel

grub2-editenv list

:!: The first item in the list denote entry '0'

Set default kernel

grub2-set-default 0

:!: This will set first entry in list to default

Change kenel command line option

One should change

vi /etc/default/grub
# Change your addition / removal here
GRUB_CMDLINE_LINUX

Grubby way to change

grubby --set-default /boot/vmlinuz-4.18.0-240.15.1.el8_3.x86_64