changing Projected Total in opportunity

I'm trying to figure out how or if we can change the Projected Total on an opportunity after the opportunity is created. I can set any total I want when I first create the opportunity, but when I go back in to edit it based on new information gathered about the prospect's interest in various services, NS won't let me change the amount. Obviously, the amount varies based on probability and items selected, but we have different pricing levels for different situations, so the total calculated from the items selected is not always reliable. We need the ability to go