How do I replace a bit of text with string variable in javascript?

Question

I know this is a really simple question, but I need to replace this bit of text in a paragraph with a variable every time an even fires.

The markup looks like this

Answer 1

One example on how can be simple things made complicated :)

javascript:

// simple way:
function replace_text(text) {
    var heading = document.getElementById('heading');
    heading.innerHTML = '
' + text + '
';
}

// complicated way:
function replace_text2(text) {
    var heading = document.getElementById('heading');
    var node = heading.childNodes[0];
    while ( node && node.nodeType!=1 && node.tagName!='H1' ){
        //console.log(node.nodeType,node);
        node = node.nextSibling;
    }
    if (node) {
        node.replaceChild(document.createTextNode(text),node.childNodes[0]);
    }
}

html:

The script is here.