update Mac OS sierra
I need to update my Mac OS Sierra
iMac 27″, OS X 10.11
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Have a backup created prior to the upgrade, as that backup provides a means to revert should a problem arise with the upgrade.
Then read the following description:
How to get old versions of macOS - Apple Support
When you download the chosen macOS installer, you will want to use Safari for that download and not some other web browser, as various other web browsers have been known to fail with this particular Apple download.