To factor the expression y = x^4 - 10x^2 + 9, we can rewrite it as:

y = $x^2-9$$x^2-1$

Now, we can further factor each quadratic expression:

y = $x+3$$x-3$$x+1$$x-1$

Therefore, the factored form of the expression y = x^4 - 10x^2 + 9 is $x+3$$x-3$$x+1$$x-1$.