How can I upgrade my Mac pc

You start by telling us WHICH Mac and its SUB-MODEL. There are too many possibilities and I promise—you do not want us guessing! Gets ugly fast!

The sub-model is the key to what upgrade options, if any, you have.

Please do "About this Mac..." from your Apple menu to find an tell us Model, Sub-Model and current macOS version:

but please DO NOT post and serial numbers.

Your Mac must be at least as new as one of these to use macOS 12 Monterey:

• MacBook (Early 2016 or newer)
• MacBook Air (Early 2015 or newer)
• MacBook Pro (Early 2015 or newer)
• Mac mini (Late 2014 or newer)
• iMac (Late 2015 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013 or newer)

Older models cannot run macOS 12 Monterey

If you click on System Settings.....(the gear icon on the dock).....and then click on Software Update.....what information appears?

If the message displayed says that your iMac is up to date......there are no more updates available for your iMac.

Apparently, you have High Sierra. First, you need to check your imac model identifier to see its compatibility list, notably with 11.