Help needed

I'm trying to write a query which will extract data from tables. This extract will then be uploaded into a financial apps.
The place where I need help is on merging or replacing figures.
I am extracting data from two table Actual & Forecast. The actual figures are from Jan to August and are null to December. The forecast figures run from Jan to Dec.
Both of these will be entered into a version called Actual input. What I need is the actual figures to update the forecast figures and where there are no actual figures for the months to come it will replace null with the forecast figures.
Is this possible?
Please please help as I'm about ready to commit myself to lunacy