Yes (see here for details)

Arguments:

(required) long : itemid - the id of the item
(optional) string : title - the new item name
(optional) string : description - the new item description
(optional) string : tags - the new item tags
(required) string : publickey - the public key of an application
(required) string : token - the access token

Request Example:

http://www.bluemelon.com/api/rest/{api_sign}/
?method=bluemelon.items.update&itemid=62522&tags=cat&description=myCat&publickey={publickey}&token={token}

Response Example:

<rsp status="ok" onrequest="bluemelon.items.update">
  <content>
    <item id="62522" date="2009-04-05 11:13:13.0 CEST" filename="Lili (2).jpg" size="925653" etag="727e77582cb8a2280b5fa1485902dd4d" 
type="IMAGE" subtype="JPEG" title="Lili+%282%29.jpg"> 
      <description>myCat</description>     
      <tags>cat</tags>     
    </item>
  </content>
</rsp>

Error Codes:

4 - "Missing parameter"
5 - "Invalid parameter"
11 - "Item not found"
12 - "Application not found"
14 - "Invalid accessToken"
16 - "Access token timed out.You must relogin."
17 - "Invalid call signature size"
18 - "Invalid call signature"
20 - "Missing api_sign"
30 - "Title length must be between 1 and 50"
600 - "No write permissions"
601 - "Mismatched parameter in your request"
602 - "No read permissions"
603 - "Account recently expired or deleted"