ok,

HTML

<iframe name="ciao" src="pagina.htm"></iframe>
<input type="button" value="avanti" onclick="javascript:avanti();" />
<input type="button" value="indietro" onclick="javascript:indietro();" />

JS

<script>

function avanti(){
document.getElementsByName('ciao')[0].src = 'paginaAvanti.htm';
}

function indietro(){
document.getElementsByName('ciao')[0].src = 'paginaIndietro.htm';
}

</script>