The situation is this: When a distribution uses Grub as the boot loader then the last distribution installed will put its version of Grub into the MBR of the hard disk and that distribution will be at the top of the menu. If you want Ubuntu to be at the top of the menu then you must put the Ubunty Grub back into the MBR. We do this but loading into Ubuntu and running
Assuming that there is only one hard disk (sda) and you want Grub in the MBR of sda and not another hard disk (sdb, sdc etc.)
sudo grub-install /dev/sda
The first command updates the grub configuration file on the Ubuntu partition and detects all the OS it can find. The second command installs Grub into the MBR and it will look for its configuration file on the Ubuntu partition.
I have several installs of Ubuntu (different versions) and I use this method to keep one special Ubuntu in charge of the Grub menu. I have installed the selection of grub2-splashimages in the software centre. They get put into /usr/share/images/grub. I select one and copy it into /boot/grub and then I run those two commands and that gives me a background image for the grub menu. When that background image changes, then I know that some other installation has taken control of the Grub menu in the MBR.
You will need administrator privileges to copy an image into /boot/grub. Run
and then you will get a file manager with administrator privileges.