In a multiple row update scenario, you will need to set some time before invoking another Update function call. Here is the snippet, using BindtoAccessDataSource.aspx provided sample, I add a client side function to update the first 3 row of data:

function EditRows()
{
    var grid = ISGetObject("WebGrid1");

    //Update first 3 row
    var maxLoop = 3;
    var idx = 0;

    var intervalObj = setInterval(function()
    {
        if (idx < maxLoop)
        {
            var selectedRow = grid.RootTable.GetRow(idx);
            var cells = selectedRow.GetCells();

            cells.GetNamedItem("ContactName").SetText(selectedRow.GetCell("ContactName").Text + " " + idx, true);
            cells.GetNamedItem("Region").SetText("Region #" + idx, true);

            selectedRow.Update(false);

            idx++;
        }
        else
        {
            clearInterval(intervalObj);
            intervalObj = null;
        }

    }, 500);
}

Another suggestion would be to use WebGrid BatchUpdate feature, by setting AllowBatchUpdate = "true" under LayoutSettings and invoking AcceptAllChanges after updating all the rows. Here is the snippet:  

function EditRowsV2()
{
    var grid = ISGetObject("WebGrid1");

    //Update first 3 row
    for (var i = 0; i < 3; i++)
    {
        var selectedRow = grid.RootTable.GetRow(i);
        var cells = selectedRow.GetCells();

        cells.GetNamedItem("ContactName").SetText(selectedRow.GetCell("ContactName").Text + " " + i, true);
        cells.GetNamedItem("Region").SetText("Region #" + i, true);

        selectedRow.Update();
    }

    grid.AcceptAllChanges();
}

Thank you for the information