You cannot edit your forecasts, unless your budget has been locked/approved first - so ensure you do that before you attempt to forecast.
To forecast in your Planning grid, you'll just need to change the view to Actuals + Forecasts - you can show Actuals To any month you like, or none at all. If you show any actuals, then you will not be able to forecast under these months.
To forecast in your production trackers, you'll need to head to your Operations > Trackers > Select your tracker in question.
From there, you'll have a menu to Add Forecast Transaction, you can then enter stock movements as you see fit. Alternatively, you can use our re-forecaster tool.