I have a iMac (retina 4K 21.5 inch 2017, can I upgrade the software, its currently showing macOS High Sierra

A 2017 iMac supports up to the current macOS v12 "Monterey."

Apple's compatibility Information is near the bottom of this page:

macOS Monterey - Apple

However, if your iMac has the original mechanical hard drive, it will be quite slow with any os version higher than Mojave 10.14. Also, older hardware, such as Office 2011 or older and old version of Adobe Creative Suite/Cloud, will not run if you go past Mojave. If you have Apple's Fusion Drive or a factory solids-state drive, your 2017 should have no issues.

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

You can use this tool to see what 32-bits you have and whether you can upgrade them to a version compatible with macOS 10.15 or newer.

https://www.stclairsoft.com/Go64/index.html

Look here.

https://www.macworld.com/article/673697/what-version-of-macos-can-my-mac-run.html