how to turn on Bluetooth on iMac
How to turn on Bluetooth on iMac, without keyboard or mouse
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Without a keyboard or mouse you cannot alter the iMac settings.
The Magic Keyboard can be used as a USB keyboard by plugging it into the iMac with its Lightning charging cable.
Any basic USB mouse will probably work.
If necessary, borrow a usb keyboard and mouse from another computer, even a Windows pc.
When using a Windows keyboard on a Mac, the Windows key is used in pace of the Command key, and the Alt key in place of the Option key.