Your post does not make any sense.
I understand you have Core 2 Duo CPU in your Macbook. However, I don't know what you are trying to upgrade to. In your title you mentioned the Core i3, but in your post you stated "core2 duo13" which I don't know what that is.
In any case, the CPU is most likely soldered into place which makes it extremely difficult to replace. Additionally, the Core 2 Duo and Core i3 CPUs have different sockets so you cannot install a Core i3 CPU in your Macbook even if you could.