OSX by default uses a kernel that is placed in the root directory "/" and loads from a file named "mach_kernel".
So in order to replace the default kernel, all you have to do is to replace that file with your own kernel file. But remember, the kernel you are replacing with must be compatible with your current system otherwise you will not be able to boot.
In my case, I have several kernels in my root directory anv, voodoo, kabyl, and vanilla. I would like "anv" to my default kernel. To do that open the terminal and type following command’s :