Weak interaction
With more detail, beta decay begins with a neutron, which is made of one up quark and two down quarks. Since up quarks have a charge of +2/3, and each down quark has a charge of -1/3 this results as giving 2/3 -1/3 -1/3 = 0 charge. Due to weak force, if there are too many neutrons in an atom nucleus, one of the down quarks in one of the neutrons turns into an up quark. This would change the charge of the neutron from 0 to (2/3 +2/3 -1/3) = 1. From this, the neutron is no longer a neutron, but actually a proton ( a particle with a charge of +1).