The App Store only shows updates for the OS you are using at the moment, in your case Yosemite 10.10.5.
If you want to go to another newer OS that is called an upgrade. This can be done through the App Store using the following link.
Mac App Store is the simplest way to find and download apps for your Mac. To download apps from the Mac App Store, you need a Mac with OS X 10.6.6 or later.
When Apple releases a new version of macOS, the old version is removed from the Mac App Store. This has been the way major OS updates have been handled by Apple since the release of Mountain Lion. If you have previously downloaded an older version. On a computer running OS X Yosemite it is possible to see versions of Mac OS X in the Mac App Store. Yosemite, Mavericks and Mountain Lion can all be seen here.
High Sierra is the last/ newest OS your Mac can use. Scroll down to Section 4 and click on Get macOS High Sierra.
Make a full back up of your Mac before upgrading.
Check to see with the developers of any third party apps you use that there are updates available so they will continue to work in the new OS. Another thing you need to check is if any printers/ scanners etc you use have updated drivers and software so they too can continue to function in the new OS.
Feb 3, 2019 11:15 PM
Check compatibility
Mac App Store Os X 10.10 Yosemite Installer
You can upgrade to OS X El Capitan from OS X Snow Leopard or later on any of the following Mac models. Your Mac also needs at least 2GB of memory and 8.8GB of available storage space.
MacBook introduced in 2009 or later, plus MacBook (13-inch, Aluminum, Late 2008) MacBook Air introduced in late 2008 or later MacBook Pro introduced in mid 2007 or later Mac mini introduced in early 2009 or later iMac introduced in mid 2007 or later Mac Pro introduced in early 2008 or later Xserve models introduced in early 2009
To find your Mac model, memory, storage space, and macOS version, choose About This Mac from the Apple menu . If your Mac isn't compatible with OS X El Capitan, the installer will let you know.
Make a backup
Before installing any upgrade, it’s a good idea to back up your Mac. Time Machine makes it simple, and other backup methods are also available. Learn how to back up your Mac.
Get connected
It takes time to download and install OS X, so make sure that you have a reliable Internet connection. If you're using a Mac notebook computer, plug it into AC power.
Download OS X El Capitan
For the strongest security and latest features, find out whether you can upgrade to macOS Catalina, the latest version of macOS.
If you still need OS X El Capitan, use this link: Download OS X El Capitan. A file named InstallMacOSX.dmg will download to your Mac.
Install the macOS installer
Double-click the downloaded file to open a window showing its contents. Then double-click the file within, named InstallMacOSX.pkg.
Follow the onscreen instructions, which will guide you through the steps necessary to install.
Begin installation
After installation of the installer is complete, open the Applications folder on your Mac, then double-click the file named Install OS X El Capitan.
Click Continue and follow the onscreen instructions. You might find it easiest to begin installation in the evening so that it can complete overnight, if needed.
Allow installation to complete
Please allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart, show a progress bar, or show a blank screen several times as it installs both OS X and related updates to your Mac firmware.
Learn more
Mac Os X Yosemite Iso
OS X El Capitan won't install on top of a later version of macOS, but you can erase your disk first or install on another disk.