Change .text to .textContent to get/set the text content.
Or since you're dealing with a single text node, use .firstChild.data in the same manner.
Also, let's make sensible use of a variable, and enjoy some code reduction and eliminate redundant DOM selection by caching the result of getElementById.
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";
}