No Imac OS update

Sorry. For a 27" Late 2009 Mac, macOS 10.13.6 (High Sierra) is the end of the line. That is 7 major versions ahead of the original OS (Snow Leopard), and 6 major versions behind the current one (Sonoma).

It's becoming increasingly difficult to find new applications for High Sierra.

• Adobe and Microsoft moved on long ago.
• The developers of LIbreOffice have moved on. You can still get old, unsupported versions that will run on High Sierra, but you must dig around in the archives.
• Serif has moved on. When they released Affinity (Photo, Designer, Publisher) V2, they discontinued V1. The V1 applications were compatible with High Sierra, but the V2 versions require at least Catalina.

You may want to install Firefox 115 ESR. It is no longer getting new features, but Mozilla says that it will get "critical security updates" until September 2024.

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

You can not, because 10.13.6 High Sierra is the highest macOS that your Late 2009 iMac can run.

It’s simply too old to update. If you have a need for a more up to date version of Mac OS please plan to replace your 2009.

No further OS upgrades are compatible with that model.

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