Like this:
In your case, you would not have to go through the whole process of reinstalling Grub like that tutorial describes. Just get into the chroot (even using the GUI chroot tool would most likely work just fine), and move the backup to be the main config again.
mv /etc/default/grub.bak /etc/default/grub
Then regenerate the Grub configuration file.
sudo update-grub
As it is, the thread doesn’t have enough information to be able to see what could be going wrong. We’ll have to see the actual commands you are running and the terminal output to see if we can spot any issues with what is happening.