If you have a computer running El Capitan still I highly recommend you upgrade to a newer version if possible, or retire your computer if it cannot be upgraded. You then have to choose to get it from the Mac App Store.
Usually it will offer to upgrade you to a new version once, but if you dismiss the offer to upgrade it doesn’t persist. Normally auto-update only installs updates for your current software. Don’t assume that you’re on the latest version because your computer automatically updates itself. Not sure what version you are running? Here’s how to find out. The good news though, is that if you have a compatible computer software upgrades from Apple are free. So that means Mojave, High Sierra, and Sierra. Apply historically only patches software for the current software plus two versions prior. With yesterday’s software updates El Capitan fell off the security patch bandwagon, as expected.
