update os x yosemite

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

Yosemite was updated as far as 10.10.5 so your software update advice is correct.

However, your 2011 iMac can be upgraded as far as MacOS High Sierra 10.13.6

You can access the installer from this page. I've upgraded a 2011 iMac to High Sierra and it was a simple and successful installation.

To prepare for the upgrade read the good advice already posted by others on this forum, such as Kappy's advice in this post.