high sierra

All depends on which iMac and its year of manufacture, read the link given to you by fuzzy.

If you find your Mac can not upgrade to High Sierra then you can upgrade it to El Capitan.

How to upgrade to OS X El Capitan - Apple Support

If you are going to upgrade to High Sierra you will need to check that third party apps you use have updates available so they will continue to work, and Aldo check that any printers/ scanners you own have updated drivers and software updates so they too will continue to function.

Read this: How to upgrade to macOS High Sierra - Apple Supporthttps://support.apple.com/en-us/HT208969