An atom with too many neutrons is unstable because the strong nuclear force, which holds the nucleus together, is not strong enough to overcome the repulsive force between the protons. In order to become more stable, the atom will release a neutron in a process called beta decay.
When a neutron decays, it turns into a proton, an electron, and an antineutrino. The electron and antineutrino are emitted from the nucleus, while the proton remains in the nucleus. This changes the atomic number of the atom by 1, but the mass number stays the same.
The reason why an atom releases a neutron instead of a proton is because neutrons are more stable than protons. This is because neutrons do not have a positive charge, so they do not repel each other as strongly as protons do. Additionally, neutrons help to stabilize the nucleus by providing a "buffer" between the protons.
So, if an atom has too many neutrons, it will release a neutron in order to become more stable. This process of beta decay helps to ensure that the number of neutrons and protons in an atom is always in balance.