grub

Boot

Interrupt the boot process

If the GRUB screens are hidden during boot, hold down the SHIFT key until the menu displays. GRUB 2 searches for a depressed SHIFT key signal during boot.

Order

From How do I change the GRUB boot order?

Grub Customizer:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer