This will work in Chrome
// get your input
var input = document.getElementById('txt206451');
// get it's (first) label
var label = input.labels[0];
// change it's content
label.textContent = 'thanks'
But after looking, labels doesn't seem to be widely supported..
You can use querySelector
// get txt206451's (first) label
var label = document.querySelector('label[for="txt206451"]');
// change it's content
label.textContent = 'thanks'