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.

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

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

grub2-editenv list

The first item in the list denote entry '0'

grub2-set-default 0

This will set first entry in list to default

One should change

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

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