MacBook Air 2015 downgrade
I have a friend’s MacBook that he wants me to downgrade it, how do I downgrade to macos Sierra or High Sierra?
MacBook Air
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Create a bootable installer for macOS - Apple Support
What to do before you sell, give away, or trade in your Mac - Apple Support Point 8 is the relevant instruction for your specific task
Basically you create the Flash drive with the older OS, boot from it to install the OS then migrate data if you wish. Be sure you follow the steps precisely and have copious backups.