1, remember grub interface windows location
My interface is as follows: windows in line 3
2, select to enter the system ubuntu
3, the terminal is opened, the following command input
sudo vim /etc/default/grub
4, see the contents as shown below
GRUB_DEFAULT is the default boot entry (my windows is the third row, so here is 2, this picture is to find online)
GRUB_TIMEOUT is the start wait time (in seconds), wait for a while if you do not select the other systems, will enter the default system.
5, last updated grub
In the terminal to enter the following command:
sudo update-grub
Restart will find the default system has been set up successfully.