You can display the grub menu during boot by pressing continuosly keys as esc, tab or left shift key, but which of them works, depends on the hardware. It is easier to edit the grub file to always show the grub menu at boot so that you have easier access to it in case of problems.
Open the grub file with the text editor as administrator
gnome-text-editor admin:///etc/default/grub
Move with the arrow keys down to the following lines and change them so that they look like this. Keep all other lines untouched:
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=10
Then save the file.
Now update grub
sudo update-grub
to apply the changes. Don't forget that. Then reboot.
Now you should see the grub menu at boot with one line "Advanced options for Zorin". Select that.