try this..:)
function SetPage(url)
{
document.getElementById("myframe").setAttribute("src", url);
}
<iframe id="myframe"></iframe>
<span onclick="SetPage('MyPage1.aspx')">My Page1</span>
<span onclick="SetPage('MyPage2.aspx')">My Page2</span>
<span onclick="SetPage('MyPage3.aspx')">My Page3</span>
<span onclick="SetPage('MyPage4.aspx')">My Page4</span>