Quick button question

hi all,

hopefully just a quick & easy question.

I want to make the following code
work so that when the button is pressed
a named page is displayed in the *existing*
window. In the example below the named
page opens in a new window not the existing
window?

Any Ideas/help greatly appreciated.

<form>
<input type="button"
value="Click Here"
onclick=window.open("index.html")>

</form>