How to change the order to choose os during boot?

To change the default loading entry, you need to change the GRUB_DEFAULT as given below.

you need to edit /etc/default/grub . To do that,

Execute : sudo nano /etc/default/grub

Now change the GRUB_DEFAULT=0 to any entry you wish.

The first entry is 0 , second Entry is 1 and so on..

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After editing, update your grub by executing : sudo update-grub

If you would like to have a graphical interface, you may want to use Grub Customizer: As Tico pointed out, install it by adding the ppa first:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

This tool lets you modify grub rather safely. It allows you to re-order grub's menu entries, set the default entry, and remove an entry, etc.