Can I update my Mac from High Sierra to Catalina?

Be aware that if your Mac is qualified for Catalina, and if your current High Sierra system is installed on an HDD formatted as Mac OS Extended Journaled (the default for HDDs prior to Catalina), you will probably need to make & use a bootable USB installer in order to install Catalina. This is because your system drive will need to be reformatted as APFS before you can install Catalina.

If your Mac is qualified for Catalina, here are Apple's instructions on how to download Catalina (and other versions of macOS) and how to create a bootable installer:

• How to download macOS - Apple Support
• Create a bootable installer for macOS - Apple Support

You can also search this community or post a new question about how to do it.

Also note that Catalina requires apps to be 64 bit; older 32 bit apps will not run on Catalina and many won't even install. This includes Adobe CS apps and MS Office 2011.

Does your Mac qualify?

MacOS Catalina 10.15.7 - 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)

How to download macOS - Apple Support

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 or higher, to see which Apps will no longer work in 10.15 get Go64...

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