How to upgrade from OS X El Capitan 10.11.6 to macOS Sierra 10.14.3 [closed]
You downloaded the 'delta' Update file from Mojave 10.14.2 to 10.14.3, not the full upgrade installer.
You can only get the full installer through the App Store, & only if your machine can run Mojave.
If your machine cannot run Mojave, then see How can I download an older version of OS X/macOS? for suggestions.