How to reduce C′A′B + CAB′ to C′B + CB′?
What Boolean law / identity I can use to reduce $C'A'B+CAB'$ to $C'B+CB'$?


None.  You can't reduce $C'A'B+CAB'$ to $C'B+CB'$.  Those two Boolean expressions are not equivalent.  For example, when $A=\text{True}$, $B=\text{True}$, and $C=\text{False}$, $C'A'B+CAB'$ becomes $\text{False}$, but $C'B+CB'$ becomes $\text{True}$.