i cannot find OS X el captain in the apple store for update

Mac OS X El Capitan is no longer available for download through the App Store.

Instead it is a direct download from this link,

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download Install OS X El Capitan.

Here is what happens.

This downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan,

a mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support