Upgrade my old MAC air laptop

You cannot upgrade an Early 2014 MacBook Air past Big Sur.

Big Sur is not one of the "most recent three" supported by the likes of Microsoft and Adobe, but it is good enough to run current versions of

• The Firefox web browser
• The LibreOffice office suite
• The GNU Image Manipulation Program (GIMP)
• The commercial Affinity V2 applications (Photo, Designer, Publisher)

If you want to run applications that require a current version of macOS, you will need a newer Mac.

OS X 10.11 El Capitan is a downgrade from macOS 11 Big Sur.

And you really don't want to do that.