You can use an easy to use GUI app called grub-customizer to make your life a little easy. As the name suggests, you can do much more than just reordering Grub menu entries with it.
You can install it by:
sudo add-apt-repository ppa:danielrichter2007/grub-customizer sudo apt-get update sudo apt-get install grub-customizer
There is an article about it on this blog here. Take a look at it if you're interested. Also there is a solved thread on the Ubuntu Forums with a post that mentions this tool.
All you have to do is select the line you want to move and select de arrows to move it between de lines (generaly the 1st line is the default boot)
Here are some screenshots: