how to update my mac os x el capitan 10.11.6 to 10.12

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

If one gets an Error message in the Apple Store or no ”  Get Button “  Change the https in the page's address to macappstore. Credit to Contributor @Niel

macOS Sierra 10.12 hardware requirements

Notation for Sierra - it will download a DMG files to the Downloads Folders. Double Click it. Another windows will appear. It will have PKG file. Double Click the PKG file. This will install a New Application in Applications folder called " Install Sierra" That is the installer and double click it to commence the installation.

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Did you check the macOS Sierra 10.12 hardware requirements?

This article explains the upgrade process:

How to get old versions of macOS - Apple Support