how neutron can turn into electron and proton?

 it’s possible for a neutron suddenly to turn into a proton.during beta decay a neutron changes into a proton.

When it does this it emits an electron.  This is what a beta particle is.  It’s an electron given off when a neutron turns into a proton.

It’s best not to think of a neutron being ‘made from’ a proton and electron.  Just think that the neutron can turn into a proton and when this happens an electron is created at the same time.

So a beta particle comes from the nucleus of an atom