On Thu, 11 Dec 2014 20:48:38 +0530, p.rameshbabu@globaledgesoft.com said:

How can I change the kernel version from 3.12 to 3.5.0.45-generic?

First, figure out *why* you need to do that. There's probably a better solution.

Whether I need to download the source code for 3.5.0.45-generic and I need to compile it?

The -generic hints that it's a distro kernel for a long-gone release. If so, you probably want to just download the .rpm or .deb or whatever from the distro's repositories and install it.