also note you cannot have a GO statement inside your IF statements BEGIN /END block...

your example for part one would not be valid, it would have to be like this:

IF EXISTS(SELECT 1 FROM [ReorderS]) --what is your condition?

BEGIN

DELETE FROM [ReorderS]

INSERT INTO [ReorderS]

SELECT ITEM,NULL FROM [INV_ITEM_MAS]

UPDATE [ReorderS] SET Quantity = [PO_Qty]

FROM [PO ITEM INTERFACE] INNER JOIN [INV_ITEM_MAS]

ON [PO ITEM INTERFACE].[PO_Item]= [ReorderS].ITEM

END