I am using iMac (21.5-inch, Late 2013) model with OS X Yosemite 10.10.5, how to upgrade OS 10.11. System preference shows no updates available

I would advise you to upgrade to 10.14 Mojave. It's the last upgrade before Catalina that won't be limited by all your 32-bit apps.

Your iMac should run Mojave just fine.

How to Upgrade to  macOS 10.14 Mojave:

❗️– Backup your data. (You’ve been warned.)

Learn How to Back up your Mac with Time Machine:

click > https://support.apple.com/en-us/HT201250

📚 – Check Compatibility of your Mac with newer versions of macOS.

Learn how to upgrade macOS:

click > https://support.apple.com/en-us/HT211683

These Mac models are compatible with macOS Mojave:

• MacBook (Early 2015 or newer)

• MacBook Air (Mid 2012 or newer)

• MacBook Pro (Mid 2012 or newer)

• Mac mini (Late 2012 or newer)

• iMac (Late 2012 or newer)

• iMac Pro (2017)

• Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with a

recommended Metal-capable graphics cards.

For details about your Mac model, click the Apple  icon at the top left of your screen and choose About This Mac. 

✅ - Download the macOS 10.14 Mojave upgrade installer here:

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

(Don’t use FireFox for this link)

The download will place the "Install macOS Mojave" application in the Applications folder. Double-click the installer to begin the upgrade process.