File properties not updating using rest api

Hello Updated script and its working, just check for the library name "Documents" and the {"type":"SP.Data.Shared_x0020_DocumentsItem"} in the script.

function updateFileMetadata() {

    var def = jQuery.Deferred();
    var restSource=_spPageContextInfo.webAbsoluteUrl+ "/_api/Web/Lists/getByTitle('Documents')/Items(1)";        
    var itemPayload = {"__metadata": {"type":"SP.Data.DocumentsItem"},"Title":"Updated"};
    var dfd = jQuery.Deferred();
    jQuery.ajax(
    {
        url: restSource,
        method: "POST",
        contentType: "application/json;odata=verbose",
        data: JSON.stringify(itemPayload),
        headers:
            {
                "Accept": "application/json;odata=verbose",
                "X-RequestDigest": jQuery('#__REQUESTDIGEST').val(),
                "X-HTTP-Method":"MERGE",
                "If-Match": "*"

            },
        success: function (data) {
            alert("Success");
            dfd.resolve(data);
        },
        error: function (err) {

            dfd.reject(err);
            alert("Error:");
        }
    });
    return dfd.promise();
}

Tested the snippet in my side and it can update the library Title field:

<script src="http://code.jquery.com/jquery-latest.js" type="text/javascript"></script>
<script type="text/javascript">
updateFileMetadata();
function updateFileMetadata() {
        var def = jQuery.Deferred();
        var restSource= "http://sp/_api/Web/Lists/getByTitle('Documents')/Items(11)";        
        var itemPayload = {"__metadata": {"type":"SP.Data.Shared_x0020_DocumentsItem"},"Title":"Updated"};
        var dfd = jQuery.Deferred();
        $.ajax(
        {
            url: restSource,
            method: "POST",
            contentType: "application/json;odata=verbose",
            data: JSON.stringify(itemPayload),
            headers:
                {
                    "Accept": "application/json;odata=verbose",
                    "X-RequestDigest": $('#__REQUESTDIGEST').val(),
                    "X-HTTP-Method":"MERGE",
                    "If-Match": "*"

                },
            success: function (data) {
                alert("Success");
                dfd.resolve(data);
            },
            error: function (err) {
                dfd.reject(err);
                alert("Error");
            }
        });
        return dfd.promise();
    }
</script>

Check if the item type is valid with this endpoint:

http://sp/_api/Web/Lists/getByTitle('Documents')/Items(11)