Need to upgrade my Mac OS High Sierra

Ventura is still one of the "most recent three" that vendors like Microsoft and Adobe support, and still gets current Safari updates. Everything before Ventura has fallen off that list.

Back up your current system (I would suggest not just having a backup, but having a bootable external disk which you could use to recover if something goes wrong).

Then see: How to download and install macOS - Apple Support

If you upgrade to Catalina or later,

• 32-bit applications will break
• iTunes will go away, replaced by (Music, TV, Podcasts, Books) and Finder-based iPhone/iPad/iPod management
• You will be able to run current versions of Firefox and LibreOffice .

Most 2017 iMacs can run up to Ventura Mac OS 13.

The iMac Pro from 2017 can run up to Sequoia MacOS 15.