Code:
            $(function() {
                $(".vote").click(function() 
                {
                    var id = $(this).attr("id");
                    var name = $(this).attr("name");
                    var parent = $(this);
                    if(name=='up')
                    {
                        var idtrim = id.replace('-up','');
                        var dataString = 'id='+ idtrim ;
                        var oppid = idtrim + '-down';
                        $(this).fadeIn(200).html('<img src="dot.gif" align="absmiddle">');
                        alert (dataString);
 $.ajax({
                            type: "POST",
                            dataType: "json",
                            url: "updown/up_vote.php",
                            data: dataString,
                            cache: false,
                            success: function(data)
                            {
                                parent.html(data.upvalue);
                                document.getElementById(oppid).innerHTML(data.downvalue);
                            }  });
                    }
When I execute this code I get an error:

TypeError: document.getElementById(...).innerHTML is not a function
[Break On This Error]

document.getElementById(oppid).innerHTML(data.downvalue);

I have tried setting a variable and then changing html (as in the parent.html line above my error:
Code:
var opposite = document.getElementById(oppid);
opposite.html(data.downvalue);
which gives this error:
TypeError: opposite.html is not a function
[Break On This Error]

opposite.html(data.downvalue);

What am I doing wrong?
Thanks,