Upgrade to High Sierra on iMac

If you have a 2009, 2010 or 2011 iMac it could well be that High Sierra is end of the line

Go to the App Store>software update or to system preferences>software update

If it is stating "your Mac is up to date" then it is end of the line and you cannot update any further

If you select About This Mac in the Apple () menu, what does it tell you about the model and hardware model year (or time of year) of your iMac?

• If your 21.5" iMac is from Late 2009 through Late 2011, High Sierra is the end of the line.
• If it is from Late 2012 through Late 2013, Catalina is the end of the line.
• If it is from Mid 2014, Big Sur is the end of the line.
• If it is from Late 2015, Monterey is the end of the line.
• If it is from 2017, Ventura is the end of the line.
• It cannot be the one from 2019, as that one cannot run anything earlier than Mojave.

Best case is that you have one of the 21.5" 2017 iMacs – since Ventura is one of the "most recent three" versions of macOS that is supported by vendors like Adobe and Microsoft, and probably will be for close to a year longer.

How to download and install macOS - Apple Support

If you upgrade to Catalina or later,

• 32-bit applications will stop working
• iTunes will go away, replaced by separate applications (Music, TV, Podcasts, Books) and the Finder
• You will be able to run current versions of FireFox, LibreOffice, GIMP, and the commercial Affinity V2 programs – Photo, Designer, Publisher.

That depends on the year model of your iMac. see > Identify your iMac model - Apple Support

Note that the "Newest compatible operating system" is listed for each model.

If your iMac qualifies for an upgrade. see > Update macOS on Mac - Apple Support