how do i update my iMac (Retina 5K, 27-inch, mid 2015)

Back up your data, check the application compatibility table, click here, and then on the download link for a newer OS; your computer's maximum one is Monterey. Alternatively, use an older version of Chrome or a product such as Brave instead.

Catalina and above won't run 32-bit software and split the functions of iTunes up; if you choose one of them, use Go64 as well.

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Google – Chrome browser system requirements

According to Google, Chrome requires macOS Monterey or higher.