How can I upgrade my mid-2011 iMac from El Capitan to a more recent OS?

You may want to first upgrade to High Sierra and then install another browser such as Chrome; the latest version will still run on 10.11 but that is the last macOS it will run on. See: https://support.google.com/chrome/a/answer/7100626?hl=en

A mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to get old versions of macOS - Apple Support