how can I update mac os from High Sierra 10.13.6 version to 10.15?

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

Suggestion ( Friendly ) for future posting on the ASC Forums - do not included personal information like the Serial Number as it appears in above picture. Often times, the Moderator will remove the image plus since it was visible I was about to check you configuration as well as Apple Coverage for service.

This computer Qualifies for both Big Sur macOS 11 and Monterey macOS 12

Tips Big Sir Upgrade - worth reading before attempting the upGrade to Big Sur

Tips Monterey Upgrade - - worth reading before attempting the upGrade to Monterey

just to add to the excellent (and correct) advice already provided, the links for older macOS are temporarily broken. you can be patient and wait for them to be fixed, or wait until someone provides the terminal command to enable you to download catalina with that method.

Hi please can you help I'm struggling, so I have an imac with High Sierra and it has never updated from here? I have tried to go to apple and upgrade but just cant see how to do it.. I'm a designer and just bought a macbook pro 2021 so i was thinking to link it up to use newest apps throught aht? Is my mac too old to upgrade? HELP

[Image Edited by Moderator to Remove Serial Number]