update OS from High Sierra to newer one.

The model and year of your Mac will determine which macOS you can upgrade to,

read the following links.

How to upgrade to macOS Mojave - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

Upgrading to a new/ newer OS is free.

If you are wanting to upgrade from High Sierra to Catalina, first do a system backup. Then open the App Store, search for Catalina and click the Download button. Catalina is free.