How to upgrade MacOs to and older version desktop?
I am trying to install a new program, but it don't let me because I have an older version software and can not find a right MacOs version available for my desktop
iMac 27″, macOS 10.13
What's the model year of your iMac? The model year of an iMac is available at Apple menu > About This Mac.
An early 2009 iMac can be upgraded to El Capitan 10.11.6 but no later version
A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version.
A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later.
A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later.
A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later.
A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later.
For all of the above, see instructions in: How to download and install macOS - Apple Support
A 2019 or later iMac can be upgraded to the current version of Sonoma macOS 14. See:How to upgrade to macOS Sonoma - Apple Support
Also be aware that if you upgrade to Catalina 10.15 or later you will also need to update any non-Apple 32-bit apps. See: https://www.stclairsoft.com/Go64/
or
https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html