If you have chosen the dual boot, the new Ubuntu OS is ordered as the primary OS as below.
If you want to change the booting order, you can do it by the following commands.
$ sudo -s
[sudo] password for xxxx: yyyy
# cd /etc/grub.d
# ls
00_header 10_linux 20_memtest86+ 30_uefi-firmware 41_custom
05_debian_theme 20_linux_xen 30_os-prober 40_custom README
# mv 10_linux 11_linux
# mv 30_os_prober 10_os_prober
# update-grub2
Generating grub configuration file ...
Found Linux Mint 18 Sarah (18) on /dev/sda1
Found linux image: /boot/vmlinuz-4.4.0-31-generic
Found initrd image: /boot/initrd.img-4.4.0-31-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
done
# reboot