I have an iMac (27-inch, Late 2013) with High Sierra OS 10.13.6, what OS is the newest that is compatible with this iMac.
What is the newest OS for my iMac 27-inch late 2013 computer?
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An iMac from 2013 can upgrade as far as macOS Catalina 10.15.7.
For Mojave or Catalina,
Open Safari and click on this link,
How to get older versions of macOS
Go to the Use Safari to download macOS section and click on the OS you want to install.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you are considering upgrading to Catalina be warned it doe not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this free app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are offering 64bit updates.