how do I update why operating system

See if you can upgrade to 10.14, or 10.15, see sys requirements...

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

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

macOS Catalina - Technical Specifications

General Requirements

• OS X 10.9 or later
• 4GB of memory
• 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
• Some features require an Apple ID; terms apply.
• Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:

• 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 or newer)

https://support.apple.com/en-us/HT210190

https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12

How to upgrade to macOS Catalina – Apple Support

And caution on 10.15, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

1. To download macOS software updates, choose Apple menu > SystemPreferences, then click Software Update. Tip: You can also choose Apple menu > About This Mac, then click Software Update.
2. To update software downloaded from the App Store, choose Apple menu > App Store, then click Updates.

https://support.apple.com/guide/mac-help/get-macos-updates-mchlpx1065/mac

How to upgrade what OS to what OS on what device?

Assuming you want to upgrade to Catalina: How to upgrade to macOS Catalina - Apple Support