function toggleText(button_id)
{
var el = document.getElementById(button_id);
if (el.firstChild.data == "Lock")
{
el.firstChild.data = "Unlock";
}
else
{
el.firstChild.data = "Lock";
}
}
//Or even more compact like this:
function toggleText(button_id) {
var text = document.getElementById(button_id).firstChild;
text.data = text.data == "Lock" ? "Unlock" : "Lock";
}