hi how can i update my imac

as Paul Conaway has said, we would more info to determine the most recent macOS your iMac can run.

to check compatibility with big sur and to get the download click on How to upgrade to macOS Big Sur - Apple Support

if you can't run big sur, check compatibility for older macOS and to get the downloads, click on How to get old versions of macOS - Apple Support

Mohammandreza245, it depends on how old your computer is and which operating system version you can upgrade to. Please let us know which iMac you have. If you go to the Apple menu and Select "About this Mac" in the window that appears it should show you something like this iMac (Retina 5K, 27-inch, 2020) - Let us know what your's shows. Also in the same window it should show you what your current operating system version is. Let us know which version you are currently running.

If you have a recent macOS installed you can launch system preferences and select the Software update and will look for available OS updates for you.

I hope this helps.

Paul