A free neutron decays into a proton but a free proton does not decay into neutron. This is because

Mass of the neutron is $m_n=1.67493\times {10}^{-27} \mathrm{kg}$,

mass of the proton is $m_p=1.67262\times {10}^{-27} \mathrm{kg}$.

Proton's rest mass is less than that of a neutron's rest mass. As particle can not decay to a state of greater mass, hence the proton does not decay to neutron outside nucleus whereas the neutron can decay into a proton though $\beta$-decay.