To have another option in the grub menu be used as default, you may change GRUB_DEFAULT= below to a higher number.
You may also select options from submenus; I can't seem to find any recent accepted answers showing that though; I THINK it should be ...=2>1 for second menu entry and first suboption there.
Here: show the menu 3 seconds, then go for first entry (note update-grub !):
$ head -n 8 /etc/default/grub
# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
# info -f grub -n 'Simple configuration'
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=3
Also note: if you change /boot/grub/grub.cfg directly, it will be rewritten at next time the OS does update-grub (e.g. as new kernel gets added).