How do I update my macOS version?

Look at the tech specs—

Typically you are always better off running the most current macOS your machine will allow—especially for the built in security features.

Old app compatibility maybe one reason to hold back— you have to do your own homework here: https://roaringapps.com/apps

32-bit app compatibility with macOS High Sierra 10.13 ... - Apple Support

https://support.apple.com/en-us/HT208436

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https://www.apple.com/macos/catalina/

macOS Catalina - Technical Specifications - Apple Support

https://support.apple.com/kb/SP803?locale=en_US

System requirements macOS 10.15 Catalina

macOS Catalina will run on all Macintosh computers that supported Mojave:[1]

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

Mac Pro: Late 2013 or newer; Mid 2010 or Mid 2012 if upgraded with a recommended Metal-capable GPU

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macOS 10.14 Mojave - Technical Specifications:

https://support.apple.com/kb/SP777

Upgrade to macOS Mojave - Official Apple Support

https://support.apple.com/macos/mojave

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

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)

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macOS 10.13 High Sierra - Technical Specifications:

https://support.apple.com/kb/SP765?locale=en_US

How to upgrade to macOS High Sierra

https://support.apple.com/en-us/HT208969

MacBook (Late 2009 or newer)

MacBook Pro (Mid 2010 or newer)

MacBook Air (Late 2010 or newer)

Mac mini (Mid 2010 or newer)

iMac (Late 2009 or newer)

Mac Pro (Mid 2010 or newer)

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If you can run macOS 10.12 Sierra, you can run macOS 10.13 High Sierra

https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

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OS X 10.11 El Capitan - Technical Specifications:

https://support.apple.com/kb/SP728

How to upgrade to OS X El Capitan - Apple Support

https://support.apple.com/en-us/HT206886

MacBook (Early 2015)

MacBook (Late 2008 Aluminum, or Early 2009 or newer)

MacBook Pro (Mid/Late 2007 or newer)

MacBook Air (Late 2008 or newer)

Mac mini (Early 2009 or newer)

iMac (Mid 2007 or newer)

Mac Pro (Early 2008 or newer)

Xserve (Early 2009)

Well, we need to know the exact model, version, year of your device to determine if you can upgrade or not.