If you wish to work on a new forecast, simply click on 'Forecasts' in the top right - far right of the menu, small font.
Click the top left green button 'Create New Forecast'.
Fill in all the details as with the original forecast you set up, making sure the name is different.
You have the option to create the forecast from scratch, or you can copy an existing forecast's values by selecting yes to the question. And then selecting the forecast you wish to copy.
Then click save.
This will then become the active forecast. Any time you wish to change the active forecast to a different forecast, just click the green 'set active' button to the right of your chosen forecast.