Question: I'm using codeigniter.Now I am upgrading CodeIgniter from 2.2.0 to 3.0.0

I need to use PhpExcel.But it shows an error like Non-existent class: CI\_Excel

Please anyone help me.

Answer: 
The system/ directory is reserved for CI, you're not supposed to modify or put anything in there.

Any custom libraries you use should go into *application/libraries/*.
change the library name As CI\_Ex.... and save it on libraries folder