why won't my computer update?
Why can't I update my computer?
iMac 27", macOS 10.13
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Perhaps these definitions will help..
10.13.5 to 10.13.6 = Update
10.13.x to 10.14.x = Upgrade
See if you can upgrade to 10.12, 10.13, 10.14, or 10.15, see sys requirements...
https://support.apple.com/kb/sp742?locale=en_US
https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12
https://support.apple.com/kb/sp765?locale=en_US
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12
https://support.apple.com/kb/SP777?locale=en_US
https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12
macOS Catalina - Technical Specifications

General Requirements
▪ OS X 10.9 or later
▪ 4GB of memory
▪ 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
▪ Some features require an Apple ID; terms apply.
▪ Some features require a compatible internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:
▪ MacBook (Early 2015 or newer)
▪ MacBook Air (Mid 2012 or newer)
▪ MacBook Pro (Mid 2012 or newer)
▪ Mac mini (Late 2012 or newer)
▪ iMac (Late 2012 or newer)
▪ iMac Pro (2017)
▪ Mac Pro (Late 2013 or newer)