Edit `/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

# default
#GRUB_TIMEOUT_STYLE=hidden
#GRUB_TIMEOUT=0

# menu
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=10

# disable splash
#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
```