After installation of Linux distro alongside with Windows 10 or other Linux distro, the GRUB boot menu will appear for you to select the operating system on every startup. Set the main option to be highlighted as your primary operating system at every first boot is definitely useful.
First of all, know what are the options available in your GRUB menu. Let's say if the GRUB menu looks like this:
- elementary OS
- Advanced options for elementary OS
- Windows 10
- Type sudo nano /etc/default/grub
- If you want Windows 10 to boot by default, look for the line GRUB_DEFAULT=0 and replace with GRUB_DEFAULT=3
- To save the file and exit, press Ctrl+O and then Ctrl+X.
- Update GRUB by typing sudo update-grub
- Done. You may reboot and see the changes in GRUB menu.
If you want to boot elementary OS by default taking the options in the GRUB menu above mentioned as example, replace GRUB_DEFAULT=0 and replace with GRUB_DEFAULT=1. The entry options are starting from 0 to 3 in sequence.