Using ExactTarget DEManager to update record?

Question

Is there a way to use ExactTarget DEManager to update a data extension record instead of only add? I have a data extension that adds subscribers to specified lists (each list is its own boolean column). If a user goes to my DEManager form and the subscriber already exists, the form will return the error url ("subscriber already exists" or something similar). Is there any way instead for the form to update the existing subscriber record instead of it returning an error? If so, is there a way to use DEManager to first check if a record exists, and if it doesn't then add, otherwise update?

Reference: the DEManager section at http://help.exacttarget.com/en-US/documentation/exacttarget/subscribers/web_collect/

<form action="http://cl.exct.net/DEManager.aspx" name="subscribeForm" method="post">
        <input type="hidden" name="_clientID" value="12345" />
        <input type="hidden" name="_deExternalKey" value="WebCollectSubs" />
        <input type="hidden" name="_action" value="add" />
        <input type="hidden" name="_returnXML" value="0" />
        <input type="hidden" name="_successURL" value="http://example.com/Success" />
        <input type="hidden" name="_errorURL" value="http://example.com/Failed" />
        Email: <input type="text" name="Email"><br />
        First Name: <input type="text" name="FirstName"><br />
        Last Name: <input type="text" name="LastName"><br />
        <input type="submit" value="Submit">
    </form>

Answer 1

DEManger does support update.

<input type="hidden" name="_action" value="update" />

For an add/update or upsert action, it is recommended to use the SOAP API data extension functionality for forms that are hosted outside of the Salesforce Marketing Cloud, or Smart Capture for forms in Marketing Cloud landing pages.

Answer 2

You have to use below code it will work fine for me.

<input type="hidden" name="_action" value="add/update" />;