Possible Duplicate:

For every matrix $A\in M_{2}( \mathbb{C}) $ there's $X\in M_{2}( \mathbb{C})$ such that $X^2=A$?

Square root of a matrix

Let $A$ be $n\times n$ matrix on $\mathbb C $. Can I find $B$ such that $B^2=A$?

If I can , how do I construct $B$?