my imac is currently on the macOS High Sierra operating system. when i go into the app store to update the OS system there is no update available. How can i update my OS to the Big Sur

that would depend on the year and size of your imac. you can check compatibility for big sur and find the download here: How to upgrade to macOS Big Sur - Apple Support.

if it's not compatible with big sur, you can check compatability for an older (but more recent than high sierra macOS) here:

How to get old versions of macOS - Apple Support

From your menu : About This Mac panel, look at the parenthesis contents of the Mac product line description. If your iMac is older than a Late-2014 model, you cannot upgrade to Big Sur. Otherwise, Big Sur is in the Mac App Store available to download.

Back up your data, check your applications for compatibility, and if your iMac is a 2014 or newer model, click on Big Sur. If not, it's too old.

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