How is a neutron converted into a proton?

Neutrons and protons consist of up and down squares, which are the smallest elementary parts we know. A proton consists of two up squares and a down square and a neutron of an up square and two down squares.

What then happens is shown in Feynman charts:

As you can see, a down-quark of the neutron is converted into an up-quark, resulting in a proton. This is done by the weak interaction W-.