How to Change Boot Order on Mac?

How do I Set OS X to Boot by Default?

Hold down the Option key when booting, and when you see the system choices.
Select your desired default boot device, then hover mouse over the uparrow ↑
Hold down the Ctrl key and you should see the ↑ icon change to a "power on" icon.

How do I Change my Default Mac to Dual Boot?

In macOS, choose Apple menu > System Preferences, then click Startup Disk.
Select the startup disk that has the default operating system you want to use.
If you want to start up using the default operating system now, click Restart.

How do I Change my Primary Boot Drive Mac?

Connect the drive you want to use as a startup disk to your computer.
From the Apple menu, select System Preferences.
Select Startup Disk.
Select the hard disk or partition you want to use as the startup disk.