How to combine Insert and Updates?

[lithium1976]

Hello I am trying to merge my insert and update Sprocs into one single insert/update Sproc -

ALTER PROCEDURE [dbo].[TEST]
(  
@bookingsItemsID as int,
@quantityRequested as int,
@stockID as int,
@bookingsID as int
)
AS
BEGIN
If @bookingsItemsId < 0
BEGIN
INSERT INTO tblBookingsItems (stockID, bookingsID)
VALUES(@stockID, @bookingsID)
END

ELSE

BEGIN 
If @bookingsItemsId > 0
DECLARE @CHANGE int

SELECT  @CHANGE = quantityRequested  -  @quantityRequested
FROM tblBookingsItems  
WHERE bookingsItemsID = @bookingsItemsID

UPDATE tblBookingsItems SET
quantityRequested = @quantityRequested 
WHERE 
bookingsItemsID = @bookingsItemsID 

UPDATE tblStock SET
numberInStock = numberInStock + @CHANGE 
WHERE 
StockID = @StockID 

end

I was previously using two buttons in vb.net, one for inserting, one for updates -

Dim insertCommand As New SqlCommand("test", conn)
        insertCommand.CommandType = CommandType.StoredProcedure
        BookingsDataAdapter.InsertCommand = insertCommand
        insertCommand.Parameters.AddWithValue("@bookingsID", Me.dgvBookings.SelectedRows(0).Cells(0).Value.ToString)
        insertCommand.Parameters.AddWithValue("@stockID", Me.dgvStock.SelectedRows(0).Cells(0).Value.ToString)
        insertCommand.ExecuteNonQuery()

   Dim updateCommand As New SqlCommand("test", conn)
        updateCommand.CommandType = CommandType.StoredProcedure
        BookingsItemsDataAdapter.UpdateCommand = updateCommand
        updateCommand.Parameters.Add("@bookingsItemsID", SqlDbType.Int, 100, "bookingsItemsID")
        updateCommand.Parameters.Add("@quantityRequested", SqlDbType.Int, 100, "quantityRequested")
        updateCommand.Parameters.Add("@stockID", SqlDbType.Int, 100, "stockid")
        updateCommand.Parameters.Add("@bookingsID", SqlDbType.Int, 100, "BookingsID")
        BookingsItemsDataAdapter.Update(ds, "tblbookingsItems")

How should I combine the two commands into one single insert/update command button? It works fine with two buttons, one for each but I would love to try using just one button for both commands. The update will update the fields QuantityRequested and NumberInStock. The insert will insert BookingsID and StockID into TblBookingsItems.