I installed SteamOS in a dual boot with Ubuntu 14.04 (I had Ubuntu before SteamOS) and my GRUB Boot loader switched over to the SteamOS version. I preferred the Ubuntu one because it was easier to customize (I know they're technically the same thing, but SteamOS is weird and doesn't format some things like Ubuntu does).
How do I specify Ubuntu's GRUB as the default GRUB Bootloader?
sudo update-grub
in a terminal. – Jonas Czech Jun 06 '16 at 15:08