I have macOS High Sierra 10.13.6 and I want to upgrade.

This depends a bit on the computer you have.

Here's link to the upgradability of some macs to some system-version:

https://support.apple.com/en-us/HT210222

Similar links exist for other versions...

Please use your favorite search-engine (or apples search-function) to look for how far you can get with your mac.

To upgrade from High Sierra, you would want to

1. Make a complete backup!
2. Make sure you have access to the backup.
3. If you use specialty-software, make sure it runs on the system you aim for.
4. open App Store, search for the system you aim for, download it and run the installer.
5. before you update, don't forget to make a backup :)

(I know it's nagging but trust me, you'll be glad to have one once you need it. Everybody sooner or later needs access to a backup.)

The system installer in fact manages pretty much everything that needs to be done. The difficulty lies in finding the right system-installer...

Upgrading a macOS depends upon the model Mac, specifications, and year. To get more information about your computer, choose:  (Apple menu) > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". There's more about this in "About System Information [Profiler] on your Mac" - https://support.apple.com/HT203001

Use the computer information, including the model year, and the links below to determine what is possible with your computer.

Click on this link to read how to get different versions of macOS - https://support.apple.com/HT211683

- The article also has links to system requirements for each OS version which you should check against your exact computer model. Note some of the model identifier articles are outdated. For any model produced in the last 5 years or so you should use the system requirements links for each OS version instead. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

- Make sure you have more RAM (memory) than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you need to use Safari to get links for High Sierra and later using the Mac App Store application to open a functioning page.

- Upgrades may take some time (up to an hour or more) to install, with restarts and blank screens.

Before upgrading, back up your data in case something goes wrong. See this support document: Back up your Mac with Time Machine - https://support.apple.com/HT201250 or use a cloning tool.

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. To check for those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/

https://discussions.apple.com/thread/252735879

I've answered this question recently for another High Sierra user.