When you install a new Linux alongside another one, your computer will likely use the last installed OS’s GRUB instead of the one from the previous install.
In my case, I have installed Debian, and a few days later, a Linux Mint on the same disk. But the thing is I prefer the Debian’s GRUB because it is prettier, with a nice image background! Also, I want to keep Debian as my default choice on boot. I could have modified the boot order via Grub Customizer (I will let you Google that) but still! The Debian’s GRUB is better than the Ubuntu’s (Linux Mint’s) dark and ugly one. 😛
Step by step
To use the old install’s GRUB, use those commands from the old OS (Debian for me):
1sudo grub-install /dev/sdX
… with the X corresponding to your disk of installation. For example:
sudo grub-install /dev/sda
- Then reboot and enjoy!
I hope this helps! I have found those steps on the Ubuntu’s documentation: https://help.ubuntu.com/community/Grub2/Installing