Now i'm having yosemite in my mac and I've been trying to update my mac from yesterday.My wifi has some problem so it might be the problem to download ei capitan it disconnect each and every time when i resumed it.Finally it starts from beginning. So I'm planning to download it through offline. Please anyone shed some light on this question
There's clearly no way you can download anything offline. It sounds like you are encountering problems with the download server or your internet connection.
You could download the El Capitan installer on another more reliable Mac computer WITHOUT installing it and afterwards make a bootable USB-stick, from which you can install the OS offline.
What you need is a USB-stick drive of minimum 8GB and the free program DiskMaker X. (http://diskmakerx.com). Be advised that DiskMaker X erases the USB-stick, so don't use one with data already on it. The process of making the bootable USB-stick is pretty self explanatory once you start the program.
- Download the installer you want and then disconnect from all networks.
- Use that installer to make a USB drive that boots and installs the OS.