SUSE sca_sles15 - SUSE Certified Administrator in Enterprise Linux 15

To make the third menu entry of your GRUB2 boot menu your default menu selection, you need to do the following tasks:

Edit the /etc/default/grub file and set GRUB_DEFAULT to 2

Run the grub2-mkconfig command

The /etc/default/grub file contains various settings for the GRUB2 boot loader, such as the default menu entry, the timeout value, the theme file, etc. The GRUB_DEFAULT setting can be set to a numeric value that corresponds to the menu entry index (starting from 0), or a string value that matches the menu entry title. The grub2-mkconfig command will generate a new /boot/grub2/grub.cfg file based on the settings in /etc/default/grub and other configuration files and scripts. References: https://documentation.suse.com/sles/15-SP3/html/SLES-all/sec-grub2-config.html#sec-grub2-config-default