\(\lnot(A\leftrightarrow B)\equiv \lnot((A\rightarrow B)\wedge (B\rightarrow A))\stackrel{\text{DeMorgan}}{\equiv}\)
\(\lnot(A\rightarrow B)\vee \lnot(B\rightarrow A)\equiv\)
\( \lnot(\lnot A\vee B)\vee \lnot(\lnot B \vee A)\stackrel{\text{DeMorgan}}{\equiv}\)
\((\lnot \lnot A\wedge \lnot B)\vee(\lnot A\wedge \lnot \lnot B)\equiv\)
\((A\wedge \lnot B)\vee (\lnot A\wedge B)\), q.e.d.