If you dont have separate partition for /boot, I would recommend that you boot into the new Ubuntu install i.e the one installed later and delete the partition for older one and run
sudo update-grub afterwards. You can then reclaim that disk space by creating a new partition in it.
If you do otherwise i.e boot into the older install of Ubuntu and delete the partition for new Ubuntu installation, you may get into the situation where Grub in MBR is trying to access "boot" directory in the partition you just deleted and therefore system will not boot. In this case you will need to re-install Grub. If you really want to follow this path, you will first need to download Grub Rescue disk and after you have deleted the partition for newer Ubuntu installation and ran
update-grub, boot your system from the rescue disk and re-install Grub from there. You can find many excellent tutorials for this using Google but if you dont have any thing important in the old installation of Ubuntu, I will suggest that you stick to first approach.
To answer the updated question, I believe you just need to run "
sudo update-grub" from Ubuntu and your problem will be solved.