To download the newest version of Keynote

You'd have to go to the Mac App Store (if it's on a MacBook) and click on the "updates" tab. If Keynote can be updated, it'll show up with a button that says "update". Click on it to update it. Currently, the latest version of Keynote is version 9.2.1.

On iOS and iPadOS, your apps should update automatically.

If you really are on OS X El Capitan (10.11) then there is no Keynote version available for you, because the current version of Keynote (v9.2.1) in the Mac App Store requires either macOS Mojave (10.14), or macOS Catalina (10.15) installed beforehand.