Restriction of automorphism to a fixed field an automorphism of the fixed field? If We assumed we had a Galois group of a finite field extension $Gal(K/k)$. and a fixed field $L = K^{H}$ where $H < Gal(K/k)$. why is it true that the restriction of an automorphism in $Gal(K/k)$ an automorphism of $L$?

This is not true in general.
Consider $k = \Bbb Q, K = \Bbb Q(\sqrt[3]{2}, \zeta_3)$. The extension $K/k$ is Galois, with $\mathrm{Gal}(K/k) \cong S_3$ (since the Galois group embeds in $S_3$ and has $6$ elements). 
Let $L = \Bbb Q(\sqrt[3]{2}) = K^H$ with $H=\mathrm{Gal}(K/L)$.
The element $\sigma \in \mathrm{Gal}(K/k)$ such that 
$$\sigma(\sqrt[3]{2}) = \zeta_3 \sqrt[3]{2},\qquad
\sigma(\zeta_3)=\zeta_3$$
doesn't even restrict to a morphism $\sigma\vert_L : L \to L$. 

However, if $\sigma(L) \subset L$, then $\sigma\vert_L : L \to L$ belongs to $\mathrm{Aut}_{k\mathrm{-alg.}}(L)$, since a field morphism is injective and $L$ is finite dimensional over $k$. The condition $\sigma(L) \subset L$ holds for any $\sigma \in \mathrm{Gal}(K/k)$ when $L/k$ is a normal extension, i.e. when $H$ is a normal subgroup of $\mathrm{Gal}(K/k)$.
