With all these changes after the 1st grub, and considering how seldom I have to do it, every time it’s pain to remember. So:
/boot/grub/grub.cfg — watch, don’t touch.
/etc/default/grub — primary defaults file
GRUB_DEFAULT=0 — default menu entry by the position in the list
GRUB_DEFAULT="foo" — default menu entry by the exact name (at least they say so, didn’t try myself)
GRUB_TIMEOUT=10 — timeout
sudo update-grub2 — to rebuild the config file from various defaults files.