Need to Upgrade/Update from OS X El Capitan - HELP!

What MacOs you can upgrade to depends on your Macs model and age.

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

If you can't upgrade to Mojave try High Sierra, if you can't upgrade to that, you've had it.

Check to make sure your applications are compatible. .       

Application Compatibility

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

All Mac OS versions are found in the App Store.

Log in with your Apple ID and try to get Mojave.