First, we need to create a bootable USB drive for macOS.
Fortunately, there is a guide that covers how to do this:
Make sure you know which version of macOS you want to use.
If you want an older version of macOS (i.e. macOS Catalina or older), this guide, nor the guide above will work (because PlatformSupport.plist is only patched on macOS Big Sur and newer). Soon I will make a guide covering how to install older versions of macOS using OpenCore.
Just to be clear, if you have already followed Guide #2 in making a backup for your Mac, you should now have two USB Drives: one for your Mac's last compatible version of macOS, and another to update with OpenCore.
Also, if you had to install OpenCore on the USB drive instead of on the Mac's internal drive, you MUST install it on the same USB drive as the one with the updated macOS. You need the drive to both boot and update.
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