I have an IMac dated 2011. I am unable to update to current Safari. Help

A late 2009 through mid-2011 iMac can be upgraded to High Sierra 10.13.6 but no later version.

You cannot upgrade to the latest Safari but you can install the current version of Chrome https://support.google.com/chrome/a/answer/7100626?hl=en

or Firefox https://www.mozilla.org/en-US/firefox/93.0/system-requirements/

The way Safari is updated is by upgrading your computer's software. An upgrade is when you install a newer version, such as upgrading from Catalina to Big Sur. An update is to install a newer version of the same operating system. The reason for this is because Safari is installed as part of the operating system. It cannot normally be individually installed. Incidentally, a newer version of Safari does not always work with every web site. Sometimes it is necessary to install another browser such as Firefox or Chrome to use with certain web sites. They are individually installed and upgraded.