I am having trouble updating my Mac
My computer is instructing me to update software, but i am having difficulty navigating the system
iMac 21.5″, macOS 10.12
Yes, call if need be, but...
Start up from macOS Recovery
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
In Disk Utility>View, select Show all Devices, highlight the top left entry.
APFS won't work for Sierra or earlier, Sierra or earlier needs MacOS Extended Journaled. High Sierra won't work for APFS on earlier Macs.
What has often worked for others is to use this Restore Method instead of CMD+r keys...
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
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https://discussions.apple.com/thread/254793247?answerId=258936149022#258936149022
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https://discussions.apple.com/thread/255040953?answerId=259391356022#259391356022