Update

10.10.5 is the last update for Yosemite.

10.11 and later would be an upgrade and would be done through the App Store.

Depending on your exact year model of Mac will be what you can update to.

See here for requirements for available versions> link-> How to get old versions of macOS - Apple Support

For the current version of MacOS, 11.5.3 Big Sur, see here: link-> How to upgrade to macOS Big Sur - Apple Support

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. Make sure to use Only Safari to commence the download as Others may not work

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

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