<input type="button" onclick="func_1()" value="верхний регистр">
<input type="button" onclick="func_2()" value="нижний регистр">
<br>
<textarea rows="10" cols="45" id="text">привет</textarea></p>

<script>
function func_1(){ 
var elem = document.getElementById("text");
elem.value = elem.value.toUpperCase();
}

function func_2(){ 
var elem = document.getElementById("text");
elem.value = elem.value.toLowerCase();
}

</script>