There is lot of tutorial but I still meet some trouble, so I make a quick tutorial.
There are 5 step to change it clearly:
1.open your Terminal and input:sudo gedit /etc/default/grub
2.change the GRUB_DEFAULT=0 to GRUB_DEFAULT=4(if you want to enter the fifth option,count from 0) in grub file
3.save it
4.close it
5.input sudo update-grub
tips:
1.sudo will authority you changing the grub file, without sudo you can‘t change it and update it.
2.you need to close the grub file so you can update it.
change your default system opening option using Ubuntu
原文：http://www.cnblogs.com/SweetBeens/p/7821251.html