2 questions..

[gamer_oag]

Where do I find my Cpanel folder?

How do I upgrade from v6 to v8?

[bamasbest]

If you are a server/webhost admin .... /scripts/upcp


If you are just a cPanel enduser, you must contact your webhost.

[PWSowner]

As bamasbest said, if you are an end user, you need to contact your provider, but if you are the admin of the box, a manual cp upgrade is done in shell with:
/scripts/upcp

The cpanel files themselves are usually located in:
/usr/local/cpanel/base/frontend/[themename]

If you want to be able to use a custom skin and are an end user, you need to see if your host will put a skin directory in your directory and symlink to it.