You can adjust a Forecast to amend the expected revenue and/or to change the Forecast status simply by clicking in the appropriate column of the Forecast (Status, Revenue or Gross Margin) and entering a different figure or status, then clicking Save.

This will amend the revenue and gross margin figures shown at the top of the Forecast but will not change the amount or stage of the original Opportunity. Any adjustments made will be recorded in the History tab.