By Taylor's theorem $$f(z)=a_m(z-z_0)^m +a_{m+1}(z-z_0)^{m+1}+\cdots$$ in a neighbourhood of $z_0$ where $a_m\
e0$. Then $$f(z)=c^m (z-a_0)^mg(z)$$ for some $c$ where $g$ is holomorphic and $g(z_0)=1$. Then $g$ has a holomorphic $m$-th root near $z_0$. (Compose $g$ with a holomorphic branch of $w\mapsto w^{1/m}$ near $w=1$).