How can I replace the content of a panel with new content?

I have a regionContent panel which I add to my viewport.

How can I replace its content with new content?

    ...
    var regionContent = new Ext.Panel({
        id: 'contentArea',
        region: 'center',
        padding:'10',
        autoScroll: true,
        html: 'this is the original content'
    });

    var viewport = new Ext.Viewport({
        layout: 'border',
        items: [ regionMenu, regionContent ]
    });

    var newPanel = new Ext.Panel({
        region: 'east',
        title: 'Info Panel',
        width: 300,
        html: 'this is a panel that is added'
    });
    // regionContent.update(newPanel); //renders as javascript code ???
    // regionContent.remove(...) //how do I remove ALL CONTENT, I will not know what is in this panel to remove it specifically
    regionContent.add(newPanel); //adds to original content but does not replace it
    regionContent.doLayout();
    ...

.update() does this:

.add() does this: