Can I update a MACOs High Sierra Version 10.13.6 to a Mac OS 10.14 or later

That depends what model of mac you got.

To update to Mojave you need:

• MacBook (Early 2015 or newer)
• MacBook Air (Mid 2012 or newer)
• MacBook Pro (Mid 2012 or newer)
• Mac mini (Late 2012 or newer)
• iMac (Late 2012 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

Source:

https://support.apple.com/kb/SP777?locale=en_US

For never there will be other requirements.

You have an iMac model from 2010. for Mojave you would need a late 2012 or newer iMac.

Your mac does not support anything newer than High Sierra.

Not on a pre-2012 iMac.

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