Edit GRUB environment variables. More information: https://www.gnu.org/software/grub/manual/grub/grub.html.
grub-editenv /boot/grub/grubenv set default=Ubuntu
timeout
variable:grub-editenv /boot/grub/grubenv list timeout
saved_entry
variable to the default:grub-editenv /boot/grub/grubenv unset saved_entry
grub-editenv /boot/grub/grubenv list kernel_cmdline