You can edit the Forecast Items you have created at any time. To do so simply press the cog adjacent to the item you wish to edit.
Firstly, navigate to the Forecast Item in question. If no Forecast Items show below the line, press the triangle adjacent to the line to set them to display:
To edit the Forecast Item, press the cog adjacent to the line, then press 'Edit':
This will cause the Forecasting window to display, allowing changes to be made:
Once happy with the changes made, press 'Save Changes'. Otherwise, to cancel, press 'Cancel'.