a) Configuring GRUB
Open the /etc/default/grub file on your favorite text editor with superuser privileges and edit following value.GRUB_TIMEOUT=5
It specifies the time in seconds to wait for keyboard input before booting the default menu entry. Also, 0 means boot immediately and -1 (unset) means wait indefinitely.
b) Updating GRUB
sudo update-grub or sudo update-grub2 (Both of the commands are the same.)
It may take some time.