Q: How to change the default boot menu entry in GRUB2 used widely on Linux systems?

A: You can do it easily as root in three following steps:

1. Run update-grub program and copy entry name that you want to make default.
2. Edit /etc/default/grub and add or modify existing GRUB_DEFAULT="your_menu_entry_here".
3. Run update-grub program again to update the grub bootloader configuration.