>>How can I get a selected HTML text and replace it with my own?

    var editor1 = document.getElementById('<%=Editor1.ClientID%>');
    var editselection=editor1.GetSelection();  
    var r= editselection.createRange();

    alert(r.htmlText); // get a selected HTML text

You should use the following javaScript API instead of writing your own:

function PasteHTML(html)
{
    // get the cute editor instance
    var editor1 = document.getElementById('<%=Editor1.ClientID%>');
    
    editor1.PasteHTML(html);
}

 

Keep me posted