Using Live USB/CD
Just let's do it from a live cd environment. Just ensure the live cd is for the same architecture as the installed system. Open up a terminal. Then, mount your / partition.
sudo su
mount /dev/sdaX /mnt
Replace X with the number of partion which has your ubuntu installation.
Then, create a good environment, and use chroot:
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
mount --bind /run /mnt/run
chroot /mnt
If everything is OK, now you are inside your installed system, running inside a chrooted environment.
Uninstalling The Burg Boot Loader
To remove the Burg boot loader and restore GRUB/GRUB2, then from the terminal run this command:
sudo apt-get remove – -purge burg burg-common burg-emu burg-pc burg-themes burg-themes-common
To install GRUB, run these commands:
sudo grub-install /dev/sda
sudo update-grub
To install GRUB2, use these commands:
sudo grub-install /dev/sda
sudo update-grub2
When you're over, hit Ctrl+D to exit the chroot environment. then, carefully umount your system, and reboot
umount /mnt/dev /mnt/sys /mnt/proc /mnt/run /mnt
reboot
Uninstalling The Burg Boot Loader
If you want to remove the Burg boot loader and restore GRUB/GRUB2, then from the terminal run this command:
sudo apt-get remove – -purge burg burg-common burg-emu burg-pc burg-themes burg-themes-common
The information for above answer was taken from upubuntu.com and this answer