You mean ‘how do they exist in neutron stars without decaying?’ If yes, the answer is gravity! Gravity makes it impossible for all those neutrons to decay into protons and electrons. Think of the neutron star as a big nucleus of an element. When enough neutrons decay into electrons and protons, Pauli’s exclusion principle disallows any further states of the electrons, protons or neutrons. This makes weird stuffs to the neutrons in the neutron star, forming the degenerate neutron gas. Further decay and their ‘existence’ is preserved by the pressure created under the exclusion principle, called as neutron degeneracy pressure.
