i dont see software update under system prefrences

Software Update is located in the Apple Store in High Sierra.

If one is looking to UpGRADE to newer version of macOS, that will depend on the Age of the computer and what is is compatible to upGrade to. To see the age of computer >> Desktop >> About this Mac. Now decided what newer version of macOS desired / needed. The following Link will need to the various older versions of macOS. Any links in next page must be opened Only in Safari - FF / Chrome will not work.

Be certain to do a Time Machine Backup before upgrading and perhaps a TESTED Bootable Cone to external drive - incase things do wrong. NOTE: Starting in Catalina and Big Sur - No 32 bit applications / extension are supported.

System Preferences> Software update was not an option until macOS Mojave 10.14 was launched.

As you are on macOS High Sierra updates were handled through the App Store Updates page.

The Updates page will only inform you of updates for the OS you are currently running.

If you are looking to install an newer OS then that is called an upgrade.

Which OS you can upgrade to will be governed by which model and year your mac is.

Click on the  top left of your screen click on About This Mac to find out the year of your mac.

For macOS Mojave or Catalina,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to Check Compatibility to see if your mac qualifies for the OS upgrade you want.

Then go to the Download OS section and click on the OS you want to install.

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.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you want to or are able to install Big Sur, open Safari and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

If you are considering upgrading to Catalina or Big Sur be warned they do 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/ then contact the developers of the software to see

if they are offering 64bit updates.