Let $D$ be a relatively compact $C^2$ domain in a complex manifold $X$ of dimension $n$. Assume that $H^1(D,\Theta)$ vanishes, where $\Theta$ is the sheaf of germs of holomorphic tangent fields of $D$. Suppose that the Levi-form of the boundary $b D$ has at least $3$ negative eigenvalues or at least $n-1$ positive eigenvalues pointwise. We will show that if a formally integrable  almost complex structure $H$ of the Holder class $C^r$ with $r>5/2$ on $D$ is sufficiently close to the complex structure on $D$, there is a embedding $F$ from $D$ into $X$ that transforms the almost complex structure into the complex structure on $F(D)$, where  $F $ has class $C^s$ for all $s<r+1/2$. This result was due to R. Hamilton in the 1970s when both $b D$ and $H$ are of class $C^\infty$.