<script>
    function zamiana()
    {
        var text1 = document.getElementById('t1');
        var text2 = document.getElementById('t2');
        text2.value = text1.value.replace(/a/g, "b");
    }

</script>

<form>
    <p>T1: <input type="text" id="t1" value="Ala ma kota, kot ma"></p>
    <p>T2: <input type="text" id="t2"></p>
    <p><input type="button" value="zamiana" onclick="zamiana()"></p>
</form>