how can you upgrade old iMac

How old? The following gives the latest possible upgrade for some iMac model years.

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

An early 2009 iMac can be upgraded to El Capitan 10.11.6 but no later version. See: How to download macOS - Apple Support

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to download macOS - Apple Support

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to download macOS - Apple Support

A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download macOS - Apple Support

A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12. See:How to download macOS - Apple Support

A 2017 or later iMac can be upgraded to the current version of Ventura macOS 13. See: How to upgrade to macOS Ventura - Apple Support

In  > About This Mac, what is the iMac year?

If you post that model year, somebody here can provide the available upgrades, if any.

An iMac (Late 2012 or newer) can get to macOS 10.14, for instance.

Or if you want to look for yourself: How to download macOS - Apple Support