I'm reading Forster's Lectures on Riemann surfaces. The proof of theorem 2.1(you don't need to know what it is) of the book uses the following fact.
Let $g(z)$ be a holomorphic function defined on a neighborhood of $0$ in $\mathbb{C}$ such that $g(0) \neq 0$. Let $k > 0$ be an integer. Then there exists a holomorphic function defined on a neighborhood of $0$ such that $h^k = g$.
How do we prove this?