Forecast figures are drawn from the Budget that is referred to in your Report settings. To edit, you should first navigate to:

Budgets & Cashflow > Budgets & Actuals

Then, select the Organisation and Budget version that you want to edit.

Set the dates for the Period you wish to update and overtype the figures as required. If your budget is being consolidated, it is a Business Unit Budget and figures will need to be updated by selecting the relevant Business unit on the left side, and then edited in the grid.