using DotNetBrowser;
using DotNetBrowser.DOM;
using DotNetBrowser.Events;

...

Browser browser = BrowserFactory.Create();
browser.FinishLoadingFrameEvent += delegate(object sender, FinishLoadingEventArgs e)
{
    if (e.IsMainFrame)
    {
        DOMDocument document = e.Browser.GetDocument();
        DOMElement button = document.GetElementById("button-id");
        button.Children[0].NodeValue = "New Button Name";
    }
};

browser.LoadHTML("<html><body><button id='button-id'>Button</button></body></html>");