First, an observation: the line highlighted below is correct as per your logic? This query will set the dstatus and downloadname fields in a row as $_POST['dstatuschange'] and $_POST['dlname'] if downloadname field is the 'dlname' string. Maybe you wanted to put the $_POST['dlname'] variable in your WHERE condition?
Second, wordpress custom POSTs work much better if you use the standard wordpress way to manage them. Look at this answer to see how to handle properly a custom form submission.