Question

how a neutron is converted onto positron and proton

Answer 1

Do you mean electron and proton?

Answer 2

\[n \rightarrow p + e + \bar \nu_{e}\]

internally

\[d \rightarrow u + e + \bar \nu _{e}\]

where a down quark is converted into an up quark , also giving an electron and an electron-antineutrino in the process. It happens due to decay due to the weak nuclear force.

Answer 3

him1618 is right, \[udd \rightarrow uud + W^- \rightarrow uud + e^- + anti electronneutrino\]