A free neutron decays to a proton but a free proton does not decay to a neutron. This is because
neutron is a composite particle made of a proton and an electron whereas proton is a fundamental particle
neutron is an uncharged particle whereas proton is a charged particle
neutron has large rest mass than the proton
weak forces can operate in a neutron but not in a proton
neutron has large rest mass than the proton.
A nucleus is made up of two fundamental particles- neutrons and protons. If a nucleus has more number of neutrons than what is needed to have stability, then neutrons decay into protons and if there's an excess of protons, then they decay to form neutrons. Since a neutron has larger rest mass than a proton, the Q-value of its decay reaction is positive and a free neutron decays to a proton, while an isolated proton cannot decay to a neutron as the Q-value of its decay reaction is negative. Hence, it is physically not possible.