old macbook tells me no updates available but I am on OS X el capitan version 10.11.6

Click on the Apple top left of your screen, then click on About this Mac.

Depending on your Macs model and its year will determine which macOS you can upgrade to,

read the following links.

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Only if it's a 2012 or newer model.

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