I have question on Theorem $4$ of Evan's book page $340$ or ($6.3$ Regularity).
Evan wrote in the seventh part:
Choose $s >0$ so small that the half-ball $U' := B^0(0,s) \cap \{y_n >0\}$ lies in $\Phi(U \cap B(x^0,r)).$ Set $V' := B^0(0,s/2) \cap \{y_n >0\}.$ Finally define
$$u'(y) := u(\Psi(y)) \quad (y \in U').$$ It is straightforward to check $u' \in H^1 (U')$ and $u' =0 \quad \text{on }\partial U' \cap \{y_n = 0\}. $
I do not know how he get $u' =0 \quad \text{on }\partial U' $. Can someone explain it to me? Thank you.