I think that you are asking only for assistance in changing which OS GRUB boots by default on your system. This has been asked and answered previously. Please take a look at this question and its answers:
How do I set Windows to boot as the default in the boot loader?
GRUB_DEFAULT= to the numeric position in the GRUB menu of the OS you want to boot is a "less special" approach. This method can break if new entries are added to your GRUB menu.
Two other approaches you could use which might serve you better are:
GRUB_DEFAULT= to the title of the menu entry you want as your default. That method is described in this answer to the question above.
GRUB_DEFAULT= so the default is to boot the operating system you booted the last time (also referred to as the "saved method"). This is discussed in this answer.
Please check the version of GRUB used on your computer
According to this section of the Ubuntu GRUB documentation, you can check the current version installed on your system ... from the command line by opening a Terminal and entering the following:
Grub 2 should display a version number of 1.96 or later. Grub Legacy is version 0.97.
Please also take a look at the version information displayed on your GRUB boot menu. It's pretty easy to find, but I provided an image to illustrate anyway. :-)
Another thing you could do which would help is to install the Boot-Repair tool and use it to
Create a Bootinfo summary.
To do this you would first install the tool using
apt-get. The terminal commands to do this are
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
- When this completes (successfully), enter the command
boot-repair to start the tool.
- After a slight delay,
boot-repair will prompt you to download the newest version. Since you just downloaded & installed the tool there is no need for this so answer
boot-repair requests to install the
pastebinit package, respond with
- The tool will now scan your system and (eventually) display the window shown in the image below. For now just click on the box/button to
Create a Bootinfo summary. This will collect information about your system's boot configuration, but will not make any changes.
When the bootinfo summary has been created,
boot-repair will display a message containing a URL like this:
http://paste.ubuntu.com/123456/. Please add this URL to your question. The information this link points to will allow us to better understand how GRUB is set up on your computer.