This is how to edit grub and change the boot order.
https://help.ubuntu.com/community/GrubHowto/ChangeDefaultOS

Note: in this example they are changing it so Windows XP is the default, but the
principle is the same.

In your situation, you want to change the default number to whatever the
corresponding number for Ubuntu is. If Mint was the default it would have been 0
and Ubuntu 1. Just change the Default 0 to default 1 and save it.

An alternative is to cut and paste the Ubuntu section and place it ahead of Mint
section and leave the default at 0. Either should work. Don't forget to save
your changes.