The App Store only shows updates for the OS you are using at the moment, in your case Yosemite 10.10.5.
To download and install OS X Yosemite: Click the App Store icon on the Dock. The App Store will open. Locate and select OS X Yosemite, then click Free Upgrade. How to Install Mac OS X Yosemite on VMware on Windows; How to install Mac OS X Yosemite 10.10 on VirtualBox on Windows; Download macOS X Yosemite 10.10 Image for VMware & VirtualBox. The image file of macOS X Yosemite is created only to install in VMware and VirtualBox. Therefore, the file is completely compressed with Winrar and 7Zip.
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.
OS X Yosemite Security Updates 10.10.5 free download for Mac. With help of using these features, you can get a useful process to your Mac device using the following features as well. If you really need an earlier version of Yosemite such as 10. Very shortly, in this article, we show 3 possible ways to download Yosemite.
Mac Os X Yosemite Download Iso Bootable
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.