As described here (ubuntuhandbook.org – Boot into text console ubuntu) you need to edit /etc/default/grub to have the next boot end up in text mode. In summary you will set these parameters:
GRUB_CMDLINE_LINUX_DEFAULT="text"
GRUB_TERMINAL=console
After saving changes you need to run:
sudo update-grub
sudo systemctl set-default multi-user.target
Undoing text-mode
To undo sudo systemctl set-default multi-user.target simply type
sudo systemctl set-default graphical.target