which is the latest update can I use in macOS High Sierra

Hello, shivamanjani.

The latest version update of High Sierra is 10.13.6.

To move beyond that you will have to upgrade to Mojave, Catalina or Big Sur IF your Mac hardware is compatible.

The macOS version you can upgrade to depends on the model and year of your Mac.

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 10.14 Mojave:

• 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; Mid 2010 and Mid 2012 models with a recommended Metal-capable graphics card: https://support.apple.com/en-us/HT208898

These Mac models are compatible with macOS 10.15 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)

These Mac models are compatible with macOS 11 Big Sur :

• MacBook (2015 or newer)

• MacBook Air (2013 or newer)

• MacBook Pro (Late 2013 or newer)

• Mac mini (2014 or newer)

• iMac (2014 or newer)

• iMac Pro (2017 or newer)

• Mac Pro (2013 or newer)

macOS High Sierra was last updated to version 10.13.6 probably in 2018

Open the App Store click on Updates.

Or do you mean upgrading the OS of your mac to a newer version.

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.

If you find your mac can upgrade to the newer OS's like macOS Mojave 10.14, macOS Catalina 10.15, 

macOs Big Sur 11.4 I would recommend you upgrade to macOS High Sierra 10.13 first.

For macOS Mojave or macOS 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.