how do i upgrade from yosemite to el capitan

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

Here you can find the installer How to get old versions of macOS - Apple Support

Being on Yosemite, you can simply run the installer and it'll update to El Cap.

Your Mac must be compatible, but seems like everything that runs Yosemite also runs El Cap https://eshop.macsales.com/guides/Mac_OS_X_Compatibility