how do i upgrade to mojave os

Your Mac cannot update to Mojave.

The most you can update to is High Sierra.

See here: How to upgrade to macOS High Sierra - Apple Support

You need to download the update through the Mac App Store App, not Software Update.

Mojave would be an Upgrade instead of an Update, but yoyucan't run that OS...

macOS Mojave - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. 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 recommended Metal-capable graphics cards)