There are lots of tutorials but I still met some troubles, so I make a quick tutorial.
There are 5 clearly steps to change your turning on default system:
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.