When is the spin of an electron changing? An electron can have a half spin up or down. The up spin can become a down spin to lose his weak charge. But when are electrons changing their spin?
 A: The spin of the electron is unique in the sense that the electron moving through a magnetic field is always deflected in the same way: If the thumb points in the direction of the magnetic field and the next finger points along the direction of the electron's movement, then the third finger (which is perpendicular to the other two fingers) points in the direction of the deflection.

When is the spin of an electron changing?

What you are asking about relates to the spins of paired electrons in an atom. Pauli's exclusion principle is the expression for a phenomenon that has been found experimentally (empirically): Two electrons in an atom cannot have the same four quantum numbers, at least the spin quantum number must be different.
Since the spin and magnetic dipole of the electron are coupled (they are parallel or antiparallel in all subatomic particles and the antiparticles), it may help - just for your imagination - that two paired electrons are tiny bar magnets and these magnets are aligned antiparallel.
A: If initially you had the spin projection in a certin (eigen)state and then you apply a spin-dependent interaction, the spin wave function becomes a superposition of up and down eigenstates. Measurements will find the up-state with a probability $P_{\text{up}}$ and the down-state with the probability $P_{\text{down}}=1-P_{\text{up}}$, the mean value being between up and down.
