can i update my high sierra 10.13.6

1. Only if you need to use software which requires a higher OS.

2. Back up your data, check your applications for compatibility, and depending on which iMac model you have, click on Mojave, Catalina, Big Sur, or Monterey. it needs to be a 2012 or newer one to be upgraded.

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Choose About This Mac from the Apple menu.

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Click on Mojave or Catalina above. If you choose Catalina, it won't run any 32-bit software.

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Use Safari as other may not work.

Safari will open a redirect and open the Apple Store where the download can commence