There are two ways to enter a project forecast.
Depending on whether you entered a total or contributing project forecast, the relevant forecast row will automatically calculate revised figures. For example, let's say that the total Commit forecasted amount is 10,000 USD, with sub-project forecasts contributing 1,000 USD. If you enter 12,000 USD for the Commit value on the This Forecast row, the total forecast amount will increase to 13,000 USD after you click Save.
The forecast can now be viewed and edited on the Services Forecasts page, assuming you have permissions.