How do I update my iMac to macOS Ventura?

What is the model year of your iMac?

The model year of an iMac is available at Apple menu > About This Mac.

A late 2015 or mid 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. 

A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. 

For all of the above, see instructions in: How to download and install macOS - Apple Support

A 2019 or later iMac can be upgraded to the current version of Sequoia macOS 15. See: How to upgrade to macOS Sequoia - Apple Support

Minor point but did want to mention it, Macs do not run IOS they run Mac OS. IOS is for iPhones and there is also iPad IOS.

Please let us know the model year of the iMac, that is critical to determine if your Mac can run Ventura or not. If you do not know how to tell, in the upper left of the display is an Apple icon, click that and then select About this Mac and the window that opens will tell you the model year of the iMac.