Check the order of the desired entry

juergen@ubuntu:~$ cat /boot/grub/grub.cfg | grep menuentry

Edit /etc/default/grub

juergen@ubuntu:~$ sudo vi /etc/default/grub

And in that file set GRUB_DEFAULT to for example 2 if the 3rd entry should be the default (because here we start counting with 0).

GRUB_DEFAULT=2

Update grub.

juergen@ubuntu:~$ sudo update-grub