upgrade software from el Capitan
el Capitan expired before i had opportunity to upgrade to mojave. My recent attempts to upgrade to Mojave bring error message. my mac air 13 is 2011 version
MacBook Air 13″, OS X 10.11
A MacBook Air (11-inch, Mid 2011) cannot run Mojave (10.14) or later. You can upgrade to High Sierra (10.13).
How to download and install macOS - Apple Support
Once you are on High Sierra, you may want to upgrade to iTunes 12.8.3.
Download iTunes 12.8.3 for Mac
The version of Safari in High Sierra is still pretty old, so you may want to download a third-party Web browser. While the latest version of Firefox no longer runs on High Sierra, I believe that High Sierra can run Firefox ESR (Extended Service Release), which will probably continue to receive security patches until some time in 2024.
https://www.mozilla.org/en-US/firefox/new/
The Google Chrome Web browser might be another possibility.